Almost 90% of the chemical pulp is produced using kraft pulping method and most of the produced kraft pulp is made into bleached pulps. Therefore, in a conventional kraft pulp, a small amount of lignin found in the pulp, which is called as residual lignin, has to be removed with subsequent bleaching operations. However, bleaching is an expensive operation and results in water pollution in pulp mills. When residual lignin is removed by extending the kraft pulping to lower kappa numbers, a significant yield loss occurs due to the degradation and loss of undesirable wood components, cellulose and hemicellulose.
Increasing the yield of kraft pulp is very desirable because pulp yield is an important economic factor in pulping. The economic benefits with higher pulp yield are increased production, reduced chemical cost and less wood consumption. In kraft pulping, research has focused on minimizing the degradation and loss of carbohydrates for improved yield and better fiber strength by modifying the kraft pulping process.
One of these methods, polysulfide (PS) pulping, which is a modified kraft pulping
technology, results in substantial yield gains in both hardwoods and softwoods
(Jiang, 1995; Çopur, 2002). In this method, most of the yield increase
is brought about as a result of glucomannan stabilization. Also, the yield increase
is found to be proportional to the PS added to the cook. The properties of polysulfide
pulps are comparable to the kraft pulps. However, polysulfide pulps give higher
tensile strength but lower tear strength (Jiang, 1995; Pekkala, 1986) compared
to the kraft pulps.
Papermaking fibers should mechanically be refined to make them more suitable for papermaking. But refining effects differ depending on the cooking methods through which the pulps are produced. Emerton (1957) and Kibblewhite (1972) offer detailed information on the theory of refining and factors affecting the refining behavior of pulps. The tensile and burst strengths and folding endurance improve in refined sheet and this improvement is thought to be due to the increased fiber swelling and the applied stress on fibers during refining (Page, 1985). On the other hand, tear strength of a sheet always decreases with refining due to the strength attrition of individual fibers. In addition, during refining, the paper stock becomes more difficult to drain and the produced paper sheets become denser and bulkier while the porosity decreases. Limited research on polysulfide pulps showed that polysulfide pulps are easier to beat because of higher hemicelluloses content (Jameel et al., 1995).
Polysulfide pulping has been practiced widely in many mills all over the world.
Research on modified kraft pulping (Polysulfide) is an important endeavor to
improve pulp yield while maintaining pulp properties. Therefore, the objective
of this research is to study the effects of adding elemental sulfur directly
to the cooking digester in kraft cook on pulp yield. More specifically, the
study is concerned with the effects of refining on polysulfide pulp properties.
MATERIALS AND METHODS
Freshly cut logs from a Scots pine (Pinus sylvestris) were utilized as wood raw material. 4.7 L. M/K-digester was used to make kraft and polysulfide cooks. Pulping conditions are given in Table 1. The targeted H-factor was 2020. Polysulfide cook was made by adding 4% elemental sulfur based on oven dry chip weight to the digester along with the chips (added sulfur is not included in the sulfidity calculation).
After cooking, the pulps were disintegrated, washed with hot tap water (40°C) and then screened using a laboratory flat screen with a slot width of 0.006 (0.15 mm). The yield contents of the pulps and rejects were measured according to Tappi (T 210) by gravimetric measurements in the laboratory environment. The screened/unscreened yields were measured through duplicate analyses. The carbohydrate composition of the pulps was determined using the new NMR technique (Copur et al., 2003). Klason lignin was determined according to Tappi T 222.
Refining was accomplished using PFI mill in accordance with Tappi T 248 in which pulps were refined for 0, 3000, 6000 and 9000 revolutions. The freeness values of the pulps were determined according to Tappi T 227. Handsheets were prepared in accordance with relevant Tappi T 205 instead of 200 mesh screen to diminish the fine loss. In addition, applying high pressure (85 psi), set of handsheets were prepared using a standard press. Physical properties of handsheets were tested in accordance with the appropriate Tappi test method: tensile strength (T 494), burst strength (T 403), tear resistance (T 414), porosity (T 547)and folding endurance (T 511). The hygroscopic behaviors of fluffy pulps and handsheets (standard and high pressure) were monitored in a controlled humidity chamber. The weight of the samples were recorded when samples reached to constant weight which took 4 days.
RESULTS AND DISCUSSION
Conventional kraft and polysulfide pulps cooked under the same conditions resulted in variations in kappa number and pulp yield (Table 2). The variations could be due to the 4% elemental sulfur addition directly to the cooking digester in polysulfide pulping. On the other hand, the use of polysulfide lowered the kappa number while stabilizing the carbohydrates by oxidizing reducing end groups (Jiang, 1992). But contrary to the finding of Dillen and Noreus (1967), in this study, polysulfide was found to give a higher kappa number than that of corresponding kraft pulp (Table 2) consistent with what Jameel et al. (1995) observed. The possible reasons for this observation could be that the hydroxide that is used for delignification decreases quickly since the polysulfide sulfur reacts with hydroxide and oxidizes organic material. Moreover, it is known that higher sulphidity accelerates the delignification rate more when the alkali charge is low (Paavilainen, 1989). In other words, high sulphidity at high alkali charge may lower the delignificaiton for polysulfide pulp resulting in higher kappa number as observed in this study. As a result, if low kappa numbers are desired with polysulfide pulping, the active alkali must be increased or sulphidity must be decreased in cooks.
In terms of pulping yield, in this study, the yield of polysulfide pulp was found to be 3.8% higher compared to kraft method. Yield increase in polysulfide pulping depends on two main factors: the amount of added sulfur and the resultant pulp kappa number. In literature, yield increases of 1.5% (Jiang, 1995), 4.5% and 11% (Dillen and Noreus, 1967) were reported when 2%, 3%and 12% sulfur was added in polysulfide pulping, respectively. The percentage of yield increase in this study was lower than that of 4.5% reported by Dillen and Noreus (Dillen and Noreus, 1967). This result could be explained by higher delignification rate due to the higher alkali charge and sulphidity in this study. Another possible factor for the lower pulping yield in this study could be the secondary peeling reactions taking place during polysulfide pulping which render yield loss prevention ineffective. The carbohydrate analysis on the other hand showed that yield increase by sulfur addition is brought about by the stabilization of glucomannan and xylan in polysulfide pulping (Table 2) as also observed earlier by Jiang (1995).
Results of physical and mechanical properties are summarized in Table
3. The beatability of the pulps was evaluated based on the development of
the tensile index after a given number of revolutions in a PFI mill (Table
3). The results showed that kraft pulp required higher number of PFI revolutions
and thus required higher refining energy to reach certain freeness.
||Data on pulps kraft and polysulfide
|** Arabinose+galactose+mannose+xylose+/3 mannose (glucomannan)
||Handsheet properties of pulps at three refining levels
|* Corrected based on consistency (0.2%) and temperature (2°C),
** Sheffield porosity tester with orifice # 1. Values are averages of wire
and top sides
The ease of polysulfide pulp refining is ascribed to higher hemicellulose
retention in pulp, however; this might be a disadvantage due to the higher moisture
holding capacity of polysulfide pulps especially at higher humidity conditions
(Table 3). Polysulfide pulp showed higher tensile (Table
1) and burst index compared to the kraft pulp. The higher tensile index
could be explained by higher fiber flexibility leading to improved inter-fiber
bonding as a result of higher hemicellulose content (Table 2).
On the other hand, higher burst index of polysulfide pulp could be explained
by the higher kappa number of the initial brownstock as also observed by Parthsarathy
et al. (1995).
On the other hand, polysulfide pulp had lower tear strength than the kraft pulp. For polysulfide pulp, a constant tear value was observed but the tear value of kraft pulp decreased through refining as it was refined to higher tensile index (Table 2). It is well known that refining decreases the tear strength of pulps after reaching an initial maximum. Lower tear strength could be explained by the fact that polylsulfide pulps carry higher hemicellulose content which makes fibers more flexible. Also, polysulfide pulp, having higher pulp yield, produces fewer load-bearing fibers at certain sample weight. In addition, it is suggested that (Hakanen and Teder, 1997) higher glucomannan and lower cellulose contents may be the reason for lower tear strength of polysulfide pulp, because cellulose is the main contributor to tear strength. However, it should be noted that in this study, polysulfide pulp had higher (0.8% relative difference) cellulose content than it was reported in the literature (Hakanen and Teder, 1997). This shows that earlier explanations, such as hemicellulose content have premier effect on pulp tear strength.
In this study, the addition of 4% sulfur resulted in a higher tensile and lower
||Tear vs. tensile strength of kraft and polysulfide pulps at
different numbers of refining in PFI mill
Consequently, the strength of polysulfide pulp, which is described as tear
index at a constant tensile index, was found to be ~ 15% lower than kraft pulp,
which is inconsistent with the earlier observation (Kleppe, 1975). It seems
that refining has a positive effect on polysulfide pulp strength after 3000
(revolutions) in PFI mill (Fig. 1). The inherent fiber strength,
as found earlier (Jameel et al., 1995), is better in polysulfide pulp;
therefore, through optimized refining, it might be possible to correct the lower
strength of polysulfide pulp. In addition, similar to earlier finding (Parthsarathy
et al., 1995), in this study too, the PS pulp had lower double fold than
the kraft pulp.
The hygroscopic behavior of pulps was studied for fluffy pulps and standard
handsheets. Moisture holding ability of fluffy pulps and handsheets showed variations.
Fluffy pulps hold an average of 1.75% (relative difference) more moisture at
80% RH for both polysulfide and kraft pulps (Table 4). This
could be explained by the consolidation during the forming of handsheets which
results in less free OH groups available for moisture as a result of more hydrogen
bonds in/between fibers. The variation, on the other hand, was higher for polysulfide
(1.9% relative differences) compared to kraft (1.5% relative differences).
||Moisture of the pulps at different relative humidity
This might be due to the flexibility of the polysulfide fibers caused by higher
hemicellulose content which leads to better consolidation and less free OH groups
The results observed for the fluffy pulps and standard handsheets were similar for both polysulfide and kraft methods (Table 2). At 80% RH, polysulfide, which has more hemicellulose, held higher moisture (0.9% relative difference) compared to kraft pulp. This could be because hemicelluloses are the most hygroscopic cell wall constituents which have low degree of polymerization, include freer hydroxyl groups and are more amorphous. Along with the content of hemicellulose, the composition of the hemicellulose is likely to affect the sorption behavior too (Copur, 2002). Of hemicellulose components, Xylose was found to be the most hygroscopic (Copur, 2002; Salmen and Olsson, 1998)and since polysulfide contains higher amount of xylose, it should be expected that it hold more moisture content (Table 2).
When examining the effect of refining on moisture holding ability of fibers at various RH, the results unexpectedly showed that refining does not have any significant effect on moisture holding ability of pulps. It would be expected that as a result of refining, with increased available surface area, higher fibrillation, shorter fiber and higher fines, the moisture holding ability of fibers would increase. It seems that in chemical pulps the proportions of amorphous cellulose and hemicellulose are important factors affecting the moisture content of pulps. Refining effects, fibrillation, increasing fine contents etc., do not contribute much to moisture holding ability of chemical pulps. However, the fine content of mechanical pulps contributes the moisture holding ability considerably (Nanri and Uesaka, 1993). Stronger refining increases the sensitivity of the fibers and the resultant paper to moisture. Higher hemicellulose retention in polysulfide pulps seems to be a disadvantage due to the increased moisture holding capacity.
In this study, kraft and polysulfide pulps cooked under the same conditions showed variations in kappa number and pulp yield. Although sulfur addition in polysulfide pulping is expected to lower the kappa number and increase the yield, this study showed that polysulfide pulp gave higher kappa number that can be explained by higher sulfur content in cooking chemical that may lower the hydroxide concentration at the beginning of the cook. On the other hand, the yield increase with sulfur addition in polysulfide pulping could be due to the glucomannan and xylan retention resulting in 4% higher hemicellulose content.
Higher refining requirement with kraft pulp is due to the lower hemicellulose retention which shows the importance of pulp swelling in refining. In this study, polysulfide pulp resulted in higher tensile and burst index. Higher tensile index may be explained by higher hemicellulose retention resulting in better interfiber bonding. On the other hand, higher burst index may be due to the higher kappa number of the polysulfide pulp. Though tear index was lower for polysulfide pulp, it was constant through the refining which indicates the importance of hemicellulose. On the other hand, kraft pulp refined to higher tensile index showed lower tear index. The strength, tear index at a constant tensile index, was found to be 15% lower for polysulfide pulp compared to the kraft pulp.
Having contained higher hemicellulose content polysulfide pulp held higher
moisture indicating that moisture holding ability of pulps depends on the available
OH groups in the pulp. Refining surprisingly did not have any significant effect
on the moisture holding ability of pulps indicating that the chemical properties,
rather than the physical properties, are the main factors affecting moisture
holding ability of chemical pulps.