Page 457 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN013A-619 July 26, 2001 19:32

262 Pulp and Paper

swell and shrink more in width than in length, paper is A third type of paper machine is also utilized to a lesser
usually more dimensionally stable in the principle fiber extent: the twin wire machine. Instead of depositing a fiber
direction than in the cross-fiber direction. Other materials slurry onto a moving wire, the fiber dispersions are deliv-
may be added to paper in order to improve a particular ered into the gap of two moving wires. Machines of this
property. type remove water from both top and bottom surfaces by
pressure. Twin wire machines are capable of very high
speeds.
A. Fiber Preparation
High-speed paper machines are the result of a balance
While many factors are important in determining the prop- of the science of engineering and practical empirical ob-
erties of paper, interfiber bonding is the most significant servation. In the past, the art often preceded the science,
factor controlling strength of the sheet. The surface of but as machine speeds increase, visual observation of the
cellulose fibers is very active and is capable of forming phenomenon taking place in papermaking is virtually im-
secondary bonds (hydrogen bonds) with adjacent cellu- possible. Today, the thrust in papermaking is toward faster
lose fibers, provided that the surfaces can be brought into machine speeds while making paper lighter and bulkier,
very close contact. In paper, the driving force that brings and papermaking is becoming more of a science.
fibers into this close contact is the surface tension created
as the water is removed during drying. As fiber flexibility
C. The Use of Additives in Papermaking
increases, more surface can conform to the adjacent fiber
and a higher level of interfiber bonding can occur. The While paper can be made of wood fibers alone, little is
nature of surface bonding is also affected by the chemical actually made without some chemical addition or modifi-
makeup. Fiber surfaces high in lignin content do not bond cation. These chemical additives are used to either assist
as well as surfaces high in the amount of noncellulosic in papermaking or to give the paper certain desirable end-
carbohydrates or hemicelluloses. use qualities. These chemicals can be added at virtually
After mechanical or, to a lesser extent, chemical pulp- any step in papermaking. Some of the additives are used
ing, almost all fiber is subjected to some additional de- to influence the entire sheet properties. These chemicals
gree of mechanical action that is called synonymously are added to the pulp slurry prior to sheet formation (in-
either refining or beating. This mechanical action is im- ternal addition). When the surface properties of the sheet
portant for developing strength in paper by increasing in- also need to be altered, additives are used on the sheet
terfiber bonding. Chemical pulps are lower in lignin con- after some period of formation or drying (external addi-
tent than mechanical pulp, so refining action can more tion). A number of these chemicals serve commonly as
easily disrupt the internal cell wall material of chemical both internal or external additions.
pulp. Fibrillation is another method of increasing fiber Chemicals that aid in the papermaking process can as-
bonding by increasing the surface area of bonding. Fol- sist by increasing drainage, aid in formation or retention
lowingsuchstockpreparation,thefibersareconvertedinto of other additives, or increase wet strength. Other aids
paper. are those that reduce undesirable foaming or microbial
buildup in the system. Some of these papermaking aids
add to the pulp, but others do not and are lost during the
B. The Paper Machine
papermaking process.
Most paper today is made in continuous sheets on high-
speed cylinder or fourdrinier machines. In the cylinder
D. Process Considerations in Papermaking
machine, a wire-covered cylinder is partially submerged
in a slurry of fibers. The fibers are picked up by the wire as The processes occurring in a high-speed newsprint pa-
the cylinder revolves. The web is then removed at the top per machine have been discussed above. There are several
of the cylinder and passed into a press section. Cylinder additional considerations of note in the overall process
machines are generally made up of a series of cylinders picture. Paper for the most part is a commodity item (i.e.,
that join additional plies to the forming sheet. Most paper- production costs are more economical per unit when large
board is made on cylinder machines. Fourdrinier machines tonnages of uniform specifications are produced). Most
operate by depositing a slurry of fibers onto a moving wire. mills have a break-even point at an 85% capacity so it
The wire is supported during travel by a number of devices is vital to operate mills at design capacity. Economies of
that aid in water removal before the web passes into the scale are also found for pulp and paper mills at levels of
press section. Fourdriniers are the dominant papermaking about 1000 tonnes of paper per day for full chemical mills
machines today. They are used for most paper grades from and 200–400 tonnes of paper per day for semichemical
tissues to writing papers. or mechanical mills. Thus, the outputs of paper mills are