Ind. Eng. Chem. Res. 2010, 49, 5945–5951
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Enhanced Fiber Quality of Black Spruce (Picea mariana) Thermomechanical Pulp Fiber Through Selective Enzyme Application Marc J. Sabourin Andritz Inc., 3200 Upper Valley Pike, Springfield, Ohio 45504
Peter W. Hart* MWV Corp., 1735 Peachtree Street, NE, Atlanta, Georgia 30309
Two different enzyme applications were applied to black spruce thermomechanical pulping (TMP) in an attempt to selectively enhance the physical properties of the resulting pulp. Past studies have revealed some application difficulties between pulp and enzymes. A new method of enzyme application including fiberizing the wood chips and cooling the fiberized material prior to enzyme application was employed to maximize the fiber surface area available for enzyme reaction and to obtain optimal enzyme reaction temperatures. Fiberized chips are chips that have been destructured in a converging screw chip press followed by low energy refining to produce extremely coarse fiber pulp. TMP pulps obtained from a control sample of whole wood chips, fiberized wood treated with water, and fiberized wood with two different enzyme applications were evaluated for specific refining energy and various physical properties. Scanning electron microscopy (SEM) in conjunction with image analysis techniques was used to determine fiber wall thickness, degree of fiber fibrillation, and percent surface disruption for selected trial pulps. One monocomponent pectinase enzyme evaluated in this work was found to significantly improve various physical properties and increase the specific surface area of the resulting pulp. The multicomponent more aggressive pectinase enzyme evaluated was determined to be less effective at enhancing physical properties than the less aggressive monocomponent enzyme. The new fiberizing application method employed prior to enzyme application was found to reduce the specific refining energy requirements by 7%. When evaluated at a constant freeness, the enzyme treatment in conjunction with the new application method was found to reduce specific refining energy by about 9%. The enzyme application had only minimal impact on energy requirements. The enzyme application did significantly enhance the tensile and tear index of the resulting pulp. Dramatic increases in the amount of fibrillation and fiber surface disruption were also found to result from specific enzyme action upon the fiber. The resulting fiber wall thickness was reduced as a result of the enzyme action as well. Results from this work reveal that selective enzyme application is an effective method for enhancing specific physical properties of TMP pulps. 1. Introduction Mechanical pulps such as groundwood and refiner mechanical pulps have been traditionally employed as fillers and as methods of enhancing opacity of printing and writing papers. These fibers have the distinct advantage of extremely high pulp yields on wood of 90+% and very high (light) scattering coefficients. They also have very high bulk (inverse of density). The major limiting factors in using significant quantities of these fibers in higher quality and strength demanding papers such as woodfree printing and writing grades and coated folding board grades is that these pulps tend to exhibit low physical strength properties and require significant amounts of electrical energy to produce. Even when electricity costs are relatively low, the amount of electrical energy required to produce thermomechanical type pulps results in fairly high production costs which limit using large quantities of this material in high quality paper grades. Improvements in pulp strength have been achieved at the expense of pulp yield and opacity through the development of thermomechanical pulping (TMP) and chemithermomechanical pulping (CTMP). Both of these methods produce lower yield pulps (80-90% on a dry wood basis) but exhibit enhanced fiber strength. The strength of these fibers is still considerably lower than the strength of fully chemical fiber prepared with the kraft * To whom correspondence should be addressed. E-mail: peter.hart@ mwv.com.
or sulfite process (42-50% yield). It is well-known that interfiber bonding strength is one of the major factors in determining paper strength.1 Fiber flexibility and fiber collapsibility are also very important in interfiber bonding as they mainly determine the bonded area between two fibers. The interfiber bond strength is mainly determined by the fiber surface since bonding only occurs at the fiber surface. Therefore, the morphology of the fiber surface plays a crucial role in fiber bonding and hence the strength properties of the resulting sheet. Methods of pretreating fibers prior to TMP and CTMP have been investigated to alter fiber surface characteristics. In addition to producing inferior strength properties compared to chemical pulps, both TMP and CTMP methods still require significantly high amounts of refining energy to produce. Over the last few decades, several efforts have been made to either enhance TMP physical properties and/or decrease the specific refining energy required to manufacture TMP through the application of various pretreatments including both biological and chemical methods. 1.1. Fungal Treatment Methods. Despite the fact that the application of fungal pretreatments applied to wood chips ahead of pulping have been shown to dramatically decrease the specific refining energy required to produce TMP pulps, the use of biological agents as a method for reducing the specific energy consumption of TMP remains a relatively unpracticed technology in the pulp and paper industry. Akhtar et al. demonstrated
10.1021/ie100681e 2010 American Chemical Society Published on Web 05/17/2010
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that fungal treatment of wood chips followed by refining could reduce specific energy consumption by 20%-40% compared to conventionally produced TMP pulps.2-4 Unfortunately logistical problems were encountered when trying to implement fungal treatments commercially, including long incubation periods of over one week, sensitivity of fungi to a heterogeneous wood supply, and nonuniform fungal proliferation. Another hindrance to commercial acceptance was the problem of pulp darkening from fungal staining of wood chips.5 Earlier attempts by others using various fungi as wood chip pretreatments ahead of refining also did not obtain commercial realization.6,7 1.2. Oxalic Acid Treatment. Oxalic acid is naturally secreted at the hyphea tips (fungal roots) when fungi invades and metabolizes wood tissues. It has been hypothesized that the secretion of oxalic acid is at least partially responsible for the specific refining energy reduction obtained from fungi treatment of wood chips.8 As such, it would be reasonable to assume that impregnation of wood chips with an oxalic acid solution could impart specific energy reduction in TMP production while substantially reducing the inoculation time required to grow fungi throughout the entire chip structure. Further investigating this phenomenon in pilot scale experiments, Akhtar et al. demonstrated that oxalic acid pretreatment of wood chips could significantly reduce refiner specific energy consumption9 while reducing the total treatment time. To date, oxalic acid pretreatment of wood chips has not been commercially implemented. A requirement when pretreating wood chips with oxalic acid is to wash the residual chemical from the chips prior to refining, which serves to dampen the formation of calcium oxalate scale and related consequences on pulp mill equipment and white water systems. 1.3. Enzyme Treatment Methods. Since the late 1990s, research has focused more on enzyme treatments rather than fungi for several beneficial reasons, including shorter inoculation periods of hours instead of weeks and lesser impact on pulp brightness.10-12 In recent years, the understanding of specific enzymes and how they interact with wood components has improved. Despite advances, there remain several challenges when applying enzymes in TMP systems. These challenges manifest as the susceptibility of enzymes to denature when exposed to the high process temperatures associated with TMP processing and the need for enzymes to be transported through the wood chip to the desired reaction sites.13 Cooling of the wood substrate may be necessary in a TMP system for an enzyme formulation to be effective. A pilot-scale study by Peng and Ferritsius evaluated a pectinase enzyme solution impregnated in destructured softwood chips prior to TMP.10 A 7-10% reduction in specific refining energy consumption was reported using the pectinase treatment. The authors claimed similar to higher pulp strength properties were obtained compared to control TMP pulps, depending on the method of refining used in conjunction with the pectinase chip treatment. The highest pulp tensile index was observed using the pectinase treatment in combination with high intensity RTS refining; this observation was further validated by a higher tensile index of the long fiber fraction (16-18 mesh from a Bauer McNett Classifier) when the pulp was refined at high intensity. By way of microscopic analysis, the authors reported that pulp fibers produced from the pectinase-treated chips showed a more fibrillated structure with randomly angled microfibrils in the primary cell wall. Pulp brightness was improved by approximately 2% ISO following the pectinase treatment of Peng and Ferritsius, and the brightness gain was retained following alkaline peroxide bleaching of the pulps.6
Peng and Ferritsius reported only a marginal increase in chemical oxygen demand (COD) generation with the pectinase treatment indicating no or minimal impact on pulp yield resulting from the enzyme treatment. Increased COD would result from enzyme activity reducing long chain carbohydrate polymers of cellulose and hemicellulose to xylose, glucose, or manose monomers. Reduction in the carbohydrate chain lengths result in decreased pulp yield and increased COD demand in the filtrate. Despite promising results, the pectinase enzyme was not implemented commercially for reducing specific energy consumption in TMP operations. Work conducted by Hart et al. reported that greater than 25% energy reductions were available from alkaline peroxide mechanical pulping (APMP) of eucalyptus when preimpregnating destructured wood chips with a specific enzyme blend.13,14 Several enzyme blends were evaluated in the work by Hart et al.; different results on specific energy consumption from a significant energy reduction to none at all and different pulp properties were obtained from some of the enzyme blends. Wood fibers consist of a lignin rich middle lamella followed by a relatively thin primary wall, the P layer, a thick secondary wall which itself contains three specific layers, the S1, S2, and S3 layers, and an inner wall called the warty layer. The composition of lignin, cellulose, and various hemicelluloses varies between the different cell wall layers. Past research has indicated that processes which selectively separate fibers at the S1-S2 interface produce the highest quality mechanical pulps.15,16 Each enzyme blend used by Hart was formulated to react to varying degrees with different components of the wood structure in an effort to selectively separate the fibers at different fiber wall interfaces. Microscopic analysis revealed that pulp fibers from the enzyme blend resulting in the highest energy reduction had the highest degree of fiber wall disruption, particularly at the S1-S2 fiber interface. The optimal enzyme blend for energy reduction was also reported to have a negligible effect on pulp yield and COD. The least favorable enzyme blend, resulting in no energy reduction, had a tendency to preferentially attack the inner S2 layer structure rather than the S1-S2 interface and released more soluble carbohydrates into the resulting effluent following pulping. This work emphasized the importance of properly targeting specific fiber wall interfaces in the wood fiber structure. More recently, work by Li et al. showed that selective xylanase application to TMP and CTMP can be used to modify the surface of the pulps being produced.17 Similar to the work of Hart et al. above, Li et al. suggest that the proper application of xylanase technology to TMP may open a new venue to energy savings and pulp quality improvements through selectively splitting the fiber at the S1-S2 interface thus substantially reducing the energy requirements in secondary refining used to develop the fiber surface. Results from current investigations using enzyme treatments suggest that different wood species may require one or more combinations of selective enzymes formulated to target multiple components of the wood structure. In most enzyme studies conducted on mechanical pulps, the degree of carbohydrate dissolution is quite small, an important attribute for preserving pulp yield and the underlying economics of mechanical pulping. It is a well-understood phenomenon that enzymes react faster when applied to pulp rather than wood chips due to the increased surface area of available fibrous material. The problem, however, lies in that the potential for lowering energy consumption is considerably reduced since the pulp has already been partially refined before receiving the enzyme treatment. A primary
Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010
objective of this investigation was to evaluate enzyme treatment of fiberized softwood chips. Fiberized chips are a hybrid between that of destructured chips prepared by pressing in a chip press or pressurized compression screw device and primary refined pulp prepared in a disk or conical chip refiner with significant amounts of applied specific energy. Fiberized chips are prepared by destructuring followed by low energy refining to produce extremely coarse wood fiber that requires extensive further processing to become usable pulp fiber. In this way, a large surface area of fibrous material is exposed to the enzyme(s) while retaining a high potential for energy reduction in the final pulping process. Both an individual enzyme treatment and a multiple component enzyme blend were evaluated on fiberized softwood chips in this investigation. 2. Experimental Section Black Spruce (Picea mariana) wood chips were obtained from northern Wisconsin and shipped to the Andritz Research and Development Center in Springfield, OH. These chips were subjected to pilot scale TMP and to a new method of pretreatment prior to TMP. In an attempt to reduce the diffusion and temperature limitations associated with the use of enzymes, the chips were fiberized and cooled prior to enzyme application. Three distinct pretreatment conditions were employed. Two involved the application of different pectinase enzymes (a pure component and a blend), and the third used water as the impregnation method. The control portion of the study used whole chips. 2.1. Pulping Methodology. A new method was developed for improving enzyme activity on wood substrates, first by increasing the surface area of fibrous material for enzyme inoculation and second by controlling the temperature within the optimal range of enzyme performance. The RTFibration process consists of destructuring the spruce chips in a pressurized chip press at an inlet pressure of 1.4 bar followed by fiberization in a pressurized refiner at 1.5 bar and low specific energy with 21 °C dilution water added to cool the pulp. The spruce chips underwent RTFibration chip pretreatment to fiberize the wood, and then, the fiber was cooled for 30 min and mixed with an enzyme formulation for reaction at optimal temperature. The cooling step is important since operating above the recommended temperature range will denature the enzyme and render it inactive. Results on RTFibration pretreatment of softwoods have been previously reported.18 (i) RTFibration. The wood chips were first destructured using an Andritz pressurized RT Pressafiner chip press.19 The chips were fed via a rotary valve into a pressurized conveyor, which in turn delivered the chips to the pressurized inlet housing of the compression screw device. The retention time and pressure in the conveyor were 20 s and 1.4 bar. The wood chips were then compressed and destructured in the screw device. Four hydraulic restrictor pins on the discharge side of the press further compressed the chip plug to increase the level of destructuring. The macerated chips were then impregnated with water in an inclined conveyor and drained. The destructured chips were fiberized using an Andritz Model 36-1CP pressurized single disk refiner (91 cm diameter) at an operating pressure of 2.1 bar. The fiberized material was collected directly in drums after refining. A small amount of cold dilution water (21 °C) was added in the fiberizing step to help cool the pulp. The nomenclature used for RTFibration will be herein referred to as RTF. (ii) Enzyme Mixing and Reaction. The fiberized wood was dumped and cooled on the refiner feed deck for 30 min prior to
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mixing with water or the enzyme formulation being evaluated. The enzyme solution was added to the pulp in an Andritz Model 401 Atmospheric Refiner operating as a mixer with a wide plate gap. The treated fiber was collected in drums with plastic drum linears and allowed to react for a period of 2.5 h. The average temperature during the reaction period was 47-48 °C. Control RTF fiber was also produced using the same mixing procedure with water only for an “apples-to-apples” comparison with the enzyme-treated RTF fiber. Four refiner series were produced in this study: three on fiberized chips and one control on whole chips. Two pectinase treatments and a control water treatment were applied to the fiberized chips. Primary refining was conducted on the fiberized material using an Andritz 36-1CP pressurized refiner operating at a disk speed of 1800 rpm and pressure of 5.2 bar. The retention time prior to refining was approximately 10-15 s. High intensity directional feeding plates (Durametal pattern 36604) were used for all primary refiner runs, including the control TMP series. The nomenclature for the refiner series with fiberizer pretreatment is herein referred to as RTF-TMP. For the control TMP series, whole wood chips were refined in the primary refiner at similar operating conditions of 5.2 bar pressure and 1800 rpm refiner disk speed. The primary refined pulp was secondarily refined using an Andritz 401 atmospheric double disk refiner (91 cm diameter) using several levels of applied specific energy. The specific energy consumption (SEC) figures reported in this paper include specific energy applied at all steps including wood chip pretreatment, fiberizing, and refining. 2.2. Pulp Testing. All pulp samples were tested using standard Tappi test procedures. Shive analysis was conducted using a Pulmac Shive Analyzer equipped with a 0.10 mm screen plate. Shives are fiber bundles that are retained on a slotted screen plate. Pulp fractionation was conducted using a Bauer McNett 203C Fiber Classifier. The various pulp fractions are measured as the weight percent fiber retained between two different mesh screens. The various mesh sizes roughly correspond to fiber length. Specific surface area measurements were conduced on the whole pulp using an Andritz FiberVision Analyzer. Canadian Standard Freeness or freeness, a measure of the ability of a specified weight of fiber per specific area to impede water drainage, was measured using Tappi standard methods. Typically, increasing SEC in fiber preparation results in lower freeness numbers and decreased drainability of the resulting fiber. Additional testing for specific fiber properties was conducted on fractionated pulp samples at the PFI Research Laboratory located in Trondheim, Norway. Cross-sectional fiber characteristics of the R50 fraction (fiber retained on a 50 mesh screen) of enzyme-treated and control TMP pulps were determined. The fractionated R50 wet fibers were gently defibrated using a mild stirrer. The fibers were aligned using a specially designed apparatus described by Reme et al.20 The aligned fibers were formed into a fiber bundle and freeze-dried, which provides good separation between single fibers in the preparate. The dry fiber bundles were impregnated with epoxy and cured for 24 h. The cured preparates were cut perpendicularly to the fiber direction and ground using an abrasive paper. The surfaces were then polished and carbon-coated, and micrographs taken using scanning electron microscopy (SEM) at a resolution of 0.32 µm per pixel. Image analysis was then performed to assess fiber wall thickness, fiber cross-sectional area, and fiber circumference. Image analysis techniques were also employed to determine the amount of fiber fibrillation and the percentage of disrupted fiber surface area associated with individual fibers.
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A sedimentation specific surface area analysis was also conducted at PFI on the P16/R30 fraction (passed through a 16 mesh screen and retained on a 30 mesh screen) of selected enzyme-treated and control TMP pulps. A 2-3 g (o.d.) sample was diluted in a cylinder with deionized water and allowed to settle in a scaled measuring cylinder. The sedimentation rate of the suspension was then recorded for the initial phase of settling where the height of the fiber-water interface decreases at constant velocity as described by Wakelin.21 The test was repeated for two to three more consistency levels, after the suspension was progressively diluted. The sedimentation specific surface area (SSA) was then calculated as the inverted squareroot of the slope of the settling rate versus consistency curve. Chemical oxygen demand (COD) testing was conducted on chip and fiber filtrates up to and including primary refining using method ISO 6060-1989. Extractives content measurements were conducted on the final pulps using dichloromethane extraction, Tappi Method T204-OM-88.
Table 1. Black Spruce TMP Pulp Properties for Various Treatment Methods Evaluated at a Constant Freeness of 80 mL units mixing agent SEC bulk tensile index tear index shives scatt coefficient ISO brightness +28 mesh -200 mesh specific surface area chemical oxygen demand extractives
(kW h)/t cm3/g (N m)/g (mN m2)/g % m2/kg % % % m2/kg
none 2358 2.37 39.5 6.4 1.13 58.7 52.1 14.1 34.0 5551
water 2191 2.21 51.6 7.6 0.06 57.5 51.9 26.9 33.3 9036
E1 2144 2.15 54.8 8.0 0.07 59.5 52.0 27.0 32.8 9279
E2 2131 2.13 50.7 7.7 0.00 57.3 51.4 25.6 34.4 8558
kg/t
32.1
43.2
41.4
46.3
%
0.87
0.78
0.66
0.65
Table 2. Black Spruce TMP Pulp Properties for Three Different RTF Treatment Methods Extrapolated at a Constant Tensile Index of 53 N m/g
3. Results and Discussion Enzyme application to fiberized black spruce wood was examined for impacts in specific energy requirements for pulping, impacts upon physical properties of the resulting TMP pulp, and for specific fiber surface modifications. Two refiner series were produced using two different pectinase enzymes applied to fiberized black spruce wood chips. The procedure for enzyme mixing was described in the experimental section. The first pectinase used (E1) was a polygalacturonase (product name: Pectinex3XL) supplied by Novozymes. The enzyme protein used was separated and purified from the production organisms, Aspergillus aculeatus and Aspergillus niger. The recommended temperature range for application is 45-55 °C for this enzyme. The application dosage was 720 g/t wood. The second pectinase (E2) used (product name: Novozym863) was a more aggressive enzyme preparation produced by a selected strain of Aspergillus aculeatus. This enzyme preparation contains polygalacturonase, other pectolytic activities, and a range of hemicelluloytic activities. This enzyme preparation has the ability to disintegrate wood fiber cell wall material and works well in the temperature range of 25-50 °C. The application dosage was 830 g/t wood. Novozym863 has been previously used for modifying mechanical pulp prior to bleaching with a resultant increase in pulp brightness. The two enzyme-treated series are herein referred to as RTF(E1)-TMP and RTF(E2)-TMP. A control RTF-TMP series was produced for comparison using the same procedure except water only was added in the mixing step and is referred to as RTF(water)-TMP. Finally, a control TMP was produced with whole chips feeding the primary refining stage. 3.1. Enzyme Effect upon Specific Refining Energy. Table 1 lists the specific refining energy required to refine the black spruce wood to a TMP pulp at constant freeness levels for the control TMP, the RTF(water)-TMP, RTF(E1)-TMP, and RTF(E2)-TMP pulps. The energy values in Table 1 are reported as specific energy consumption (SEC) defined as total kilowatt hours of applied energy per metric ton of pulp processed. Various additional pulp properties are also shown in Table. 1. The results clearly show that fiberizing the wood chips ahead of primary refining was able to reduce the specific refining energy by about 7% while substantially reducing the shive content of the pulp. The application of the E1 enzyme in conjunction with fiberization of the wood chips was able to lower the specific refining energy by only 9% compared to the control pulp, while the more aggressive enzyme blend, E2,
TMP RTF-TMP RTF-TMP RTF-TMP
mixing agent scatt coefficient tear index SEC
units
RTF-TMP
RTF-TMP
RTF-TMP
M2/kg (mN m2)/g (kW h)/T
water 58.0 7.5 2248
E1 59.2 8.1 2068
E2 53.0 7.5 2277
lowered the total specific refining energy by 9.6%. From these data, it would be determined that the application of either the E1 or the E2 enzyme purely for energy reduction would not be practical for mill application. Depending upon equipment cost and the local cost and availability of electrical power, the fiberizing application may be valuable for a mill to consider. 3.2. Enzyme Impact upon Physical Properties. Each of the three RTF-TMP pulps had significantly higher pulp strength than the control whole chip TMP pulp. The results suggest that high intensity refining in the primary refining stage, using directional feeding refiner plates damaged the pulp fibers when refining whole chips. The long fiber, +28 mesh, content and pulp strength were better preserved when refining fiberized material. The RTF(E1)-TMP pulp had a significantly higher tensile index that the control pulp and a reasonably higher tensile index that the RTF(water)-TMP pulp. The RTF(E2)-TMP pulp showed a small decrease in tensile index compared to the RFT(water)TMP pulp suggesting that the more aggressive enzyme formulation may have negatively affected specific hemicellulose components in the wood structure resulting in a less effective pulp strength development. In support of this theory, the COD content of the effluent from the E2 treated pulp was higher that any other process examined. The COD of both enzyme treated pulps and of the RTF(water)-TMP pulp were all considerably higher than the COD of the control, whole chip TMP pulp. The higher COD values in these pulps resulted from the removal of some of the wood acids and soluble carbohydrates during the chip pretreatment step. The E2 enzyme treatment exhibited about a 5 kg/t higher COD content than the E1 enzyme treatment indicative of hemicelluloytic activity for the E2 enzyme. Frequently, it is desirable to refine a TMP pulp to a constant physical property value instead of a constant freeness. Table 2 compares specific energy consumption, scattering coefficient, and tear index results for each of the RTF-TMP series interpolated at a tensile index of 53 (N m)/g. The control whole chip TMP pulp was not included as it never obtained a tensile value of over 44 (N m)/g during these trials. When compared at a constant tensile index, the E1 pulp exhibited a 9% SEC reduction while the E2 treatment actually resulted in a slight energy increase over the water treated pulp. The E1 pulp also
Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010 Table 3. Number and Length of Fibrils Per Fiber for RTF(water)and RTF(E1)-TMP Pulp Fibersa % total average fibril no. of fiber no. of fibril length fiber fibers length fibrils length to fiber treatment magnification examined (µm) per fiber (µm) length enzyme water enzyme water a
200 200 500 500
16 15 13 13
819 877 31.4 15.6
11.2 6.0 11.0 5.5
106.9 92.7 74.7 56.8
139.6 59.7 165.3 61.3
Examination of electron micrographs at 200 and 500 magnification.
had a higher scattering coefficient and a higher tear index than either the water or E2 treated pulps. A high scattering coefficient at a given pulp tensile index obtained with a reduced specific energy demand are desirable attributes of the E1 treated pulps. The lower properties associated with the E2 treated pulps suggests that the more aggressive enzyme activity has a less desirable effect on the hemicellulose components of the wood. 3.3. Enzyme Impact upon the Fiber Surface. The RTF fiberizing treatment and the RTF(E1) enzyme treatment were both found to have very positive impacts on the strength properties of the resulting TMP pulps produced while reducing the shive content of the pulp and reducing the specific refining energy required to obtain a constant freeness or a constant tensile index. Frequently, strength improvements are the direct result of changes in the specific bonding area and changes in amount and length of fibrils developed on the fiber.1 Alterations or disruptions of the fiber surface have also been reported as a method for increasing pulp physical properties.17 Frequently, small changes in the specific surface area and/or fibril length and amount will result in substantial changes in physical properties of a given pulp fiber network. (i) Specific Surface Area. Both the RTF process and the enzyme activity were found to have significant levels of impact upon the fiber surface. As shown in Table 1, the application of RTF fiberizing technology increased the specific surface area of the resulting fiber by about 63%. The E1 application further increased the specific surface area by an additional 243 m2/kg or an additional 4.4% for a total increase in specific surface area of more than 67% as compared to whole chip TMP pulp. The E2 enzyme improved the specific surface area over the control whole chip TMP pulp, but it was a much smaller improvement than that resulting from the RTF(water) or
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Table 4. Evaluation of Whole and Disrupted Fiber Surface Area of Six Different Fibers Examined at 3000× Magnificationa RTF treatment
whole fiber area
disrupted fiber area
total fiber area
% of fiber surface disrupted
average percent disrupted
water water water E1 enzyme E1 enzyme E1 enzyme
360 104 421 324 145 152
114 3 11 134 113 123
473 107 432 459 258 275
24.0 2.8 2.6 29.3 43.7 44.7
9.8
a
39.2
All surface area values are in squared micrometers.
RTF(E1) treatments. As such, the E2 enzyme treated fiber was not evaluated for the rest of the surface modification examinations. (ii) Fibrillation. Electron micrographs were prepared of both RTF(water)-TMP and RTF(E1)-TMP pulp fibers at 200 and 500 magnifications. A total of 29 E1 enzyme treated fibers and 28 water treated fibers were examined. Image analysis methods were employed to determine the number and average length of fibrils associated with each fiber. The length of each fiber in the micrographs was also determined. Finally, the percent total fibril length relative to the length of the parent fiber was also calculated. These results are shown in Table 3. Representative pictures of the water treated and E1 enzyme treated fibers are shown in Figure 1. In general, the E1 enzyme treated TMP pulp had about 11 fibrils accounting for roughly 155% of the total fiber length occupied with fibrils attached to the parent fiber surface. The water treated pulp had only about 6 fibrils per fiber accounting for only 61% of the total fiber length. The E1 enzyme treatment appeared to loosen the fiber between the primary and secondary layers of the black spruce fiber resulting in several long strands of fibrils to be teased from the fiber surface. This prevalence of a substantial number of long fibril strands resulted in the very high level of specific bonding area reported above and also contributed to the improved fiber tensile and tear indexes reported in Table 1. (iii) Fiber Surface Disruption. It has been well-documented that the disruption of TMP fiber surfaces via partial peeling of the outer layer of the fiber will result in a substantial increase in carbohydrates available for fiber to fiber bonding and improve the physical properties of the resulting pulp.17 As reported by Li et al.,17 a significant portion of the fiber surface of TMP type pulps is coated with lignin resulting from the fiber-fiber split at the middle lamella during the pulping process. Disruption of this lignin coating on the surface of the fiber whether through
Figure 1. Sample SEM micrographs of (A) RTF(water)-TMP and (B) RTF(E1)-TMP pulp fibers at 500× magnification used to determine amount of fibrillation of the fiber surface.
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Figure 2. Sample SEM micrographs of (A) RTF(water)-TMP and (B) RTF(E1)-TMP pulp fibers at 3000× magnification used to determine amount of disruption of the fiber surface resulting from water and enzyme treatment.
fibrillation or fiber peeling can dramatically increase the availability of bonding carbohydrate material resulting in substantially improved strength properties. Fiber samples of the RTF(water)- and RTF(E1)-TMP pulps were examined under 3000× magnification to determine the degree of fiber surface disruption. Table 4 clearly shows that enzyme treatment resulted in a substantial disruption of the fiber surface with an average of 39.2% of the fiber surface disrupted while water treatment only disrupted 9.8% of the fiber surface. Figure 2 shows examples of the fibers treated with water and with the E1 enzyme. By disrupting a significantly larger portion of the fiber surface, the E1 enzyme treatment was able to expose more bonding rich carbohydrate portions of the fiber resulting in increased bonding area and increased fibrillation of the fiber as compared to the RTF(water) treatment. (iv) Fiber Wall Thickness. Three 70 mL freeness pulp samples (whole chip TMP, RTF(water)-TMP, and RTF(E1)TMP) were fractionated using the Bauer McNett classifier as described in the Experimental Section above. The fiber perimeter and fiber wall thickness of the R50 fraction of the fibers were determined. The control TMP fiber had a fiber perimeter of 90.1 ( 1.96 µm and a fiber wall thickness of 2.20 ( 0.05 µm. The RTF(water) and RTF(E1) pulps had fiber perimeters of 89.6 ( 1.83 and 85.6 ( 1.69, respectively. The fiber wall thickness for these fibers was determined to be 2.03 ( 0.04 and 2.06 ( 0.04 µm respectively. Because of the extensive fiber surface disruption and extreme amount of fibrillation associated with the E1 enzyme activity, the fiber perimeter of the E1 treated fiber was smaller than the fiber perimeter of either the whole TMP control pulp or the RTF(water) pulp. Finally, the three pulps were segregated by wall thickness and perimeter size. The samples were divided according to narrow and thin-walled, narrow and thick-walled, wide and thinwalled, and wide and thick-walled. Fibers with a wall thickness of less than 4 µm were considered thin-walled; fibers with a perimeter of less than 80 µm were considered to be narrow. Table 5 details the percentage of fibers falling into each of these four areas. The whole chip control TMP pulp had approximately double the wide, thick-walled fibers as compared to either of the RTF treated pulps. The E1 enzyme treated pulp had more narrow, thin-walled fibers than either the control or the RTF(water) pulp.
Table 5. Percentage of Fibers Segregated by Fiber Wall Thickness and Fiber Perimetera whole chip TMP RTF(water) RTF(E1) narrow and thin-walled fibers narrow and thick-walled fibers wide and thin-walled fibers wide and thick-walled fibers
42.8 2.6 50.4 4.2
44.0 2.3 51.3 2.4
50.9 2.5 44.4 2.2
a Thin fibers are considered to be less than 4 µm thick. Wide fibers are considered to be more than 80 µm wide.
4. Conclusions Microscopic analysis revealed the E1 enzyme treated pulp had a higher proportion of long fibers with increased external fibrillation and significantly increased fiber surface disruption. The enzyme treated TMP fiber has a smaller fiber perimeter and a larger portion of thin-walled, narrow fibers than the corresponding whole chip TMP or the RTF(water)-TMP pulps. The E1 treated pulp also exhibited a significantly higher specific surface area than either the control or RTF(water) treated pulps. All of these attributes would be expected to produce a pulp with enhanced physical properties. As expected, when compared as a constant freeness, the RTF(E1) TMP pulp had a higher tensile index and a higher tearing index than any of the other pulps evaluated. The use of RTF and enzyme technology was also able to reduce the specific refining energy required to pulp black spruce to a given freeness level or to a constant tensile index. The fiberizing impact of the RTF technology was found to have a significantly large impact on specific energy reduction while the enzyme treatment was found to be slightly additive toward specific energy reduction. The current work highlights the need to tailor the employed enzyme treatment specifically to the type of fiber being used and to the physical properties trying to be enhanced. The E1 enzyme treatment successfully enhanced the tensile and tear indexes of the resulting pulp through specific surface activity in a desirable way while the more aggressive E2 enzyme treatment was somewhat detrimental toward some of the desired pulp properties. The E1 enzyme treatment also produced a lower COD and an improved scattering coefficient as compared to the E2 enzyme treatment. In summary, the E1 pectinase enzyme was more selective than the more aggressive E2 enzyme treatment in regards to the bonding strength of TMP pulp produced from black spruce.
Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010
Acknowledgment The authors are grateful to personnel at the Andritz Research and Development Center in Springfield, Ohio, for executing the pilot trial work and analysis of pulp samples. Special thanks to personnel at PFI in Trondheim, Norway, for fiber property analysis. Thanks to Novozymes for supplying the enzymes used in this investigation. Literature Cited (1) Page, D. H. A theory for the tensile strength of paper. Tappi J. 1969, 52 (4), 674. (2) Scott, G.; Akhtar, M.; Lentz, M.; Kirk, T.; Swaney, R. New technology for papermaking: commercializing biopulping. Tappi J. 1998, 81 (11), 220. (3) Akhtar, M.; Blanchette, R.; Myers, G.; Kirk, T. An overview of biomechanical pulping research. In EnVironmentally friendly technologies for the pulp and paper industry; Young, R. A., Akhtar, M., Eds; Wiley: New York, 1998. (4) Akhtar, M.; Attridge, M. C.; Meyers, G.; Blanchette, R. Biomechanical pulping of loblolly pine chips with selected white-rot fungi. Holzforschung 1993, 47 (1), 36. (5) Agrarwal, V.; Akhtar. M. Understanding fungus induced brightness loss of biomechanical pulp. Tappi pulping/process and product quality conference, Boston, MA, November 6-9, 2000. (6) Samuelson, L.; Mjaberg, P.; Hartler, N.; Vallander, L.; Eriksson, K.-E. Influence of fungal treatment on the strength versus energy relationship in mechanical pulping. SVensk Pap. 1980, 83 (8), 221. (7) Bar-Lev, S.; Kirk, T.; Chang, H.-M. Fungal treatment can reduce energy requirements for secondary refining of TMP. Tappi J. 1982, 65 (10), 111. (8) Meyer, V.; Ruel, K.; Petit-Conil, M.; Valtat, G.; Kurek, B. Modification of the cell wall structure by oxalate associated with energy savings during mechanical pulping. 9th International conference on biotechnology in the pulp and paper industry, Durban, South Africa, October 10-14, 2004. (9) Akhtar, M.; Swaney, K.; Horn, E.; Lentz, M.; Scott, G.; Black, C.; Hartman, J.; Kirk, V. Method for producing pulp. US Patent 7,306,698, 2007. (10) Peng, F.; Ferritsius, R. Mechanical pulping with pectinase treatment of wood chips. International mechanical pulping conference, Quebec City, Quebec, June 2-5, 2003.
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ReceiVed for reView March 19, 2010 ReVised manuscript receiVed May 5, 2010 Accepted May 7, 2010 IE100681E
