Page 458 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

P1: GPB/GAM P2: GQT Final Pages
Encyclopedia of Physical Science and Technology EN013A-619 July 26, 2001 19:32

Pulp and Paper 263

enormous in terms of square meters of product produced Bristols or card stock is often thought of belonging in the
per day. Any improvement in yield in any step of manu- paperboard category, but it more rightly belongs as a paper
facture must be matched up or down the process in order product.
to benefit production. The important properties required of these papers are
If a papermill produced paper with a basis weight of surfaces suitable for printing or writing and dimen-
2
2
65 g/m (40 lb/3000 ft ) at a level of 1000 tonnes per day, sional stability. About one-half of all paper produced
a paper machine 7 m wide would have to run at speeds in is used for printing or writing. Today, much printed
excess of 1500 m/min. (almost 60 miles per hour) to pro- and information-carrying paper is handled in automated
duce this output. Most mills of this size would have more machines; therefore, the dimensional stability or the abil-
than one paper machine. Any upsets in the papermaking ity of paper to hold its dimensional tolerance is of prime
process are very costly. Any incremental improvements importance.
made in pulping yield must be matched by increasing the Paper packaging such as Kraft paper sacks and wrap-
paper machine speeds to utilize the extra furnish because ping paper, either bleached or natural, comprise about 7%
paper machines are too expensive to replace. of the total production of paper and paperboard. Competi-
One of the most energy intensive processes in paper- tion from plastics is strongly impacting paper for packag-
making is the drying of the paper sheet. Paper starts as ing. The familiar brown Kraft paper sack is rapidly being
a slurry consisting of mostly water (99%). Draining and replaced by plastic bags. Computer information storage
wet pressing will remove much of this water, but a mill of systems have not yet had the expected impact on the to-
1000 tonnes/day capacity must remove large amounts of tal amount of printing paper being made, and in some
water by evaporation using heat or mechanical energy. cases, the advent of low-cost word processing may have
The 1000 tonnes/day mill producing a web out of the increased the production of certain grades.
press at a temperature of 45 C with a solids content of 4. Newsprint: Newsprint is a commodity product with
◦
30% will need to supply about 5.5 GJ/tonnes paper to the standardized properties. The requirements for newsprint
web to remove the excess water. are low cost, printability, and runnability. Printability may
seem of obvious importance to the reader, but the concept
has many facets. The paper must accept ink without excess
E. Types of Paper
penetrationorfeathering.Thepapersurfacemustbestrong
The pulp and paper industry in the United States produced enough to resist linting or fiber pick, which can seriously
almost 100 million metric tons of paper and paperboard in affect print quality. Opacity is another important facet in
1999. This production was almost equally split between printing. Newsprint is a relatively lightweight, thin paper
paper and paperboard products, described below: and is printed on both sides. Opposite side show-through
seriously affects readability. The low cost and high opac-
1. Linerboard and Corrugating Medium: Corrugated ities found in mechanical pulp make this fiber especially
board is generally the familiar “cardboard” 3-ply lami- suited for newsprint.
nate consisting of two paperboard facers (liners) adhered Runnability is often expressed as the number of pa-
to each side of a fluted core. This construction gives high per web breaks per 100 rolls. Paper web breaks during
stiffness and strength with the benefit of low cost and press runs are very costly in terms of money and time.
weight. For special uses the construction could be varied Thus, high-speed newsprint presses also require newsprint
to include more plies or just a single face liner. Almost with good tensile properties. Although the tensions in
two-thirds of paperboard output goes into producing cor- newsprint presses are kept well below the average tensile
rugated box or container products. The linerboard is fre- strengths of the paper, breaks do occur. These are gener-
quently made of Kraft pulp, which is known for its high ally the result of some defect in the paper such as shives
strength. The fluted medium is an outlet for semichemical that are not removed from the mechanical pulp fraction of
pulp or recycled paper, which is used for stiffness rather newsprint.
than high strength. 5. Tissue: Tissue refers to a wide variety of lightweight
2. Other Paperboard: Remaining paperboard products paper from toilet tissue to napkins, towelling, wrapping,
consist of familiar products such as board for packag- andbooktissue.Therequirementsfortissuearesoftnessor
ing foodstuffs, shirt board, and tablet backs. Some pa- bulk, absorbancy, and strength. The appearance or purity
perboard is bleached for use in packaging food products. of tissue is also very important, and tissues are generally
Milk carton stock is an example of the use of bleached made of bleached pulp. Some of the chemically modi-
board. fied mechanical pulps are finding increased usage in the
3. Fine and Specialty Papers: Paper products in this tissue market. Many tissue grades require a high degree
category include writing or business and technical paper. of absorbancy while maintaining wet strength. Special