In papermaking, it is very often that more than one type (or grade) of pulps are used to develop paper sheet properties necessary for both machine runnability and requirement of the end users. In many paper grades, long fibers from softwood bleached kraft pulp are often used in combination with short fiber from hardwood bleached kraft pulp. In this combination, the long fiber component provides the strength (especially tear resistance) to the paper sheet, while the short fiber helps to improve the paper functional properties, such as sheet smoothness and formation.
It is generally accepted that fibers such as straw or SCB are not likely to be used on a 100% basis for paper manufacturing. Particularly, their poor drainability, permeability, and low tearing strength make it neces- sary to use furnish mixtures containing a certain amount of long fibers from wood.
The key element for sustainable utilization of the nonwood fibers as SCB is to understand their special qualities and how they affect the technical aspects involved. Low bulk density, short fiber, and high content of fines are the most important features. Other disadvantages include transportation and storage problems, comparatively high silica content, and very quick degradation (high losses). These disadvantages have prevented the emergence of nonwood plant fibers as a source of cost competitive pulp for both printing/writing and cellulose products particularly in regions of the world where wood supplies are adequate.
The use of nonwood fibers, however, is common in wood-limited countries [47].
The advantages, disadvantages, and related problems to nonwood pulp- ing and papermaking as SCB are discussed [48].
Hunter [49] reported that the P&P industry is a global industry producing mainly commodity products in large-scale facilities using wood as the primary fiber raw material. It is a diverse industry that produces a wide range of pulp, paper, and paperboard products to meet specific end user requirements where about 89% of all papermaking is done using wood fibers leaving only about 11% produced using a variety of nonwood fibers. When assessing nonwood plant fibers, it is also important to under- stand that there is a huge diversity of plant materials available that could be used to produce P&P. It is also important to understand that this diversity of plant fiber raw materials offers a wide range of fiber characteristics and that paper makers can use specific nonwood plant fibers to impart desir- able properties to their finished products.
Combinations of common and specialty nonwood pulps will permit the production of virtually any grade of paper to meet any quality require- ments demanded in the global market. Adding possible combinations that include wood pulp, nonwood pulp as SCB, and recycled wastepaper pulp increases the possibilities for developing paper with specific sheet properties designed to meet specific customers’ needs.
The current uses of nonwood pulps include virtually every grade of paper produced including, but not limited to:
t Printing and writing papers t Linerboard
t Corrugating medium t Newsprint
t Tissue
t Specialty papers
Specialty papers such as currency, cigarette papers, tea bags, and dielec- tric paper may be made from a furnish of 100% nonwood specialty pulps, but softwood kraft is added to provide the strength requirements to the paper. In some cases, wastepaper pulp is blended in the furnish. The non- wood portion can vary from 50 to 90% and even up to 100% depending on the grade and required quality. The possible combinations are endless and can be adjusted to meet market requirements.
Furthermore, nonwood pulps can be used as an additive to wood-based papers for a variety of reasons such as the following:
t To provide the papers with certain specific desired proper- ties—i.e. production of ultra-lightweight papers or papers with increased opacity or better bulk, etc.
t To offset higher wood costs.
t To provide an incremental increase in mill capacity in a region where woods resources are finite.
The issues surrounding the production of P&P using agricultural resi- dues and/or fiber crops are many and include both technical and economic matters. A vast body of knowledge has already been developed on the use of nonwood plant fibers. Many of the technical questions raised below have been addressed many times in many countries throughout the world by engineering consultants and equipment suppliers with expertise in the use of nonwood plant fibers.
Sadawarte [50] concluded that the main constraints for the wide- spread use of nonwood and annual plants in the P&P industry are as follows:
1. Length/diameter ratios of agricultural residue fibers are very high compared to those of wood fibers. This adversely effects the physical properties of paper.
2. The high pentosan content of some nonwood pulps reduces the power consumption in refining due to easier swelling of fibers.
3. When long fiber pulps are used along with agricultural residue pulps, separate refining of the pulps is preferable before final refining to attain homogeneity.
4. The separate refining of long fiber and wastepaper pulp requires the use of instrumentation and controls to main- tain the furnish composition. Any variation in the per- centages of short fibers and broke pulp results in hours of production loss due to breakages, particularly when bagasse and straws are the major portion of the furnish.
5. Slow drainage, a limit on the dryness at the press, poor wet strength, a tendency to blister and cockle due to rapid dry- ing, and high shrinkage are problems associated with the use of nonwood pulps in papermaking.
6. Paper machines need a wire 20–30% longer than on a wood-fiber machine to provide a drainage area large enough to maintain the required stock consistency at the headbox. Foils, vacuum foils, and additional suction boxes with large open areas on covers are other features. Mono- filament and multiple-filament synthetic fabrics are typical.
7. Even with an improved press configuration, web moisture content will be higher than with wood pulps. Web dryness improves when nonwood fiber is only 35–50% of the total furnish. In bleached printing and writing papers, the mois- ture content of the paper web is 62–65% when nonwood fiber forms 75–80% of the furnish.
8. The web should be supported in the press section to avoid breaks, as it is likely to break at the slightest change in tension. Retention of additives such as modified starches would help reduce the problem of fines and fluff.
9. These papers need an extended drying surface area at high temperatures, and there is high cross-directional shrinkage,
although camber rolls at critical points reduce the shrink- age. Dry fabrics or mesh dryers in the section following the first dryer group provide greater permeability, and evapo- rated water can be easily removed. Pocket ventilation and high-velocity hoods can also improve drying.
10. The presence of water in fiber walls is typical of these fibers.
This sets a limit to the dryness that can be achieved by increased bonding at the press and results in crushing of the sheet and marking of the felt. This can be overcome to a large extent by improved press design. A smoothing press after a wet press is essential.
11. The use of high-porosity open felts helps to achieve further dryness. Fines bound with size, and filler particles, tend to clog up the felt pores and reduce water absorbency. Regular cleaning of felts is necessary.