Unbleached pulp

Formation and Elimination of Multiple Bond Functionalities. Reactions that involve the formation and elimination of multiple bond functional groups may significantly effect the color of residual lignin in bleached and unbleached pulps. The ethylenic and carbonyl groups conjugated with phenoHc or quinoid stmctures are possible components of chromophore or leucochromophore systems that contribute to the color of lignin. 

The methods of oxidant consumption are used exclusively in the analysis of residual lignin in unbleached pulps. These procedures are all based on the common principle that lignin consumes the appHed oxidants at a much faster rate than the carbohydrates, and oxidant consumption under carefully specified conditions can be regarded as a measure of lignin concentration in the pulp. 

Two oxidants commonly used are chlorine and potassium permanganate. The Roe chlorine number, the uptake of gaseous chlorine by a known weight of unbleached pulp (ie. Technical Association of the Pulp and Paper Industry (TAPPl) Standard Method T202 ts-66) has been superseded by the simpler hypo number (ie, TAPPl Official Test Method T253 om-86), eg, chlorine consumption in treatment of the pulp with acidified sodium or calcium hypochlorite. 

Sulfite pulps have properties that are desirable for tissues and top quaHty, fine papers. Because sulfite pulping is not as versatile as kraft pulping, various options have been developed, and the choice of a specific process is dependent on individual mill situations. The unbleached pulp has high (60+) brightness compared to kraft pulp and is easily bleached. However, it is limited to select wood species. The heartwood of pine, Douglas fir, and cedars are not easily pulped. Additionally, pulps produced from hardwood have limited economic value because of low strength. 

When chlorine dioxide is used for pulp bleaching in conjunction with the Kraft (sulfide) process for chemical pulping, by-product sodium sulfate can be used as a source of makeup sulfur and sodium consumed in the chemical cycle. The demand for sodium and sulfur in pulp bleaching is related to the loss of these chemicals through carryover in unbleached pulp. As process improvements have sought to reduce pollution from pulp mills, less sodium sulfate makeup is required. The trends in pulp bleaching to increase substitution of chlorine with chlorine dioxide have caused an oversupply of sodium sulfate, so that this by-product is often regarded as waste (81). 

Basic dyestuffs are usually used for dyeing of unbleached pulp in mechanical pulp such as wrapping paper, kraft paper, box board, news, and other inexpensive packaging papers. Their strong and brilliant shades also make them suitable for calendar staining and surface coloring where lightfastness is not critical. 

Example 1 kappa (k) index of the unbleached pulp produced by a Kraft digester... 

Bleaching is defined as any process that chemically alters pulp to increase its brightness. Bleached pulps create papers that are whiter, brighter, softer, and more absorbent than unbleached pulps. Bleached pulps are used for white or light colored paper. Unbleached pulp is typically used to produce boxboard, linerboard, and grocery bags. Of the approximately 65.5 million T (72 million tons) of pulp (including recycled pulp) used in paper production in the U.S. in 2000, about 50% is for bleached pulp.1... 

Figure 5.6 Effect of refining (number of revolutions in a PFI mill) on the water adsorption isotherm of an unbleached pulp.

In addition to the use of quadrupolar splitting, the spin relaxation rate can also be used to calculate the specific surface area ratios for pulps beaten to different degrees and the results for an unbleached pulp agree closely and confirm the 250% increase in surface area measured by isotherm data (Table 5.4). 

Pulp type is also important and, in general, unbleached pulps are easier to size than bleached pulps. Pulps with a high cellulose content are extremely difficult to size and require as much as 10 times the amount of reacted AKD to produce similar levels of sizing to those of Kraft pulps. This effect is not easily understood and cannot be explained in terms of surface area. The presence of fillers also increases the amount of size required, and sizing efficiency has... 

Wood is converted to pulp by mechanical and chemical actions, which constitute the pulping process. Their selection depends upon the type of wood supply available and the pulp qualities desired. Pulps can be characterized on the basis of the unbleached pulp yields achieved by the pulping process used, i.e.. the yield of oven-drv (OD) pulp obtained from oven-dry debarked wood. 

The natural content of stilbenes is considered to be very low in spruce wood lignin and this was confirmed in the present work (Figure 5). Analysis of the aqueous extracts after mild acidic hydrolysis revealed, however, that from wood to unbleached pulps and further to bleached pulps a stepwise conversion of diarylpropane to diguaiacyl stilbene structures takes place (Figure 5). The content of diarylpropane structures was found to be 25% less in the unbleached pulp samples than in the wood with a simultaneous increase in the content of stilbene structures. After hydrogen peroxide bleaching, the content of diarylpropane structures further decreased and the... 

BLEACHED pulp. The initial experiment was followed by similar exposures of handsheets prepared from a bleached pulp (BP). Again the UF-3-filtered situation resulted in a lesser development of HAS matter (Figure 2a). The initial photochemical and subsequent thermally-induced changes in HAS matter were less extensive than in the case of the unbleached pulp (2a and b). 

Figure 1. Development of hot-alkali-soluble matter in hand-sheets of unbleached pulp during exposure to "daylight fluorescent lamps and subsequent thermal aging.

The color of unbleached pulps is caused by certain unsaturated structures (chromophores). In addition, leucochromophores, which can be converted into chromophores by air oxidation may be present in the pulp. Most of the chromophores are presumed to be derived from lignin (Fig. 7-30) although some chromophoric groups can also be introduced into the polysaccharides, for example, carbonyl groups. 

Lignin content of unbleached pulp is lower than normally. 

Disposal of spent liquors from bleach plants represents a serious environmental problem which so far has defied practical solution. While the effluents from the production of unbleached pulp have been reduced drastically by introduction of a closed pulp washing system, a corresponding... 

Unbleached pulps and groundwood pulps have poor permanency properties. Both pulps retain a high percentage of natural wood resin residuals independent of cellulose which readily decompose in the sheet. This affects color, strength, and sizing. It is generally specified that permanent paper must not contain either unbleached or groundwood fibers. 

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