Page 361 - Biosystems Engineering

338 Cha pte r Ele v e n

neutralization and destruction of excess peroxide with SO . The
2
bleaching of chemical pulp is achieved in a number of stages using
chlorine, hypochlorite, chlorine dioxide, oxygen, and peroxide. The
pulp is treated with alkali to dissolve the degradation products. After
bleaching, the pulp is passed into a beater where the surface area of
fibers is increased for water absorption and interfiber bonding. This
process is known as beating or refining, and the extent of beating in
the process determines the physical properties of the final paper. 1
A small amount of beating will result in high-tear strength with low
tensile and burst strength. Beating time is varied to produce a paper
with the desired tensile and burst strength. Several additives are
mixed during the beating process to increase the performance and
reduce the cost of paper manufacturing. A dilute solution of fibers is
then deposited onto a fine screen in the Fourdrinier system (a rotat-
ing wire-mesh cylinder connected to a vacuum pump). The sheet of
paper is then mechanically pressed and dried using steam-heated
rolls to reduce the moisture content below 10 percent.
In many applications, the paper is further treated to improve its
characteristics. These treatments include (1) calendering to improve
surface smoothness, control texture, and develop a glossy finish, (2) sizing
and coating to improve the paper’s appearance, (3) adhesive applica-
tion to increase surface strength, gloss, brightness, opacity, smooth-
ness, ink receptivity, and surface firmness, and (4) barrier coatings to
provide water vapor and oxygen barrier properties. The quality of the
final paper is measured by its physical properties including thickness,
tensile strength, stretch, bursting strength, tearing strength, stiffness,
and folding endurance. Paper is generally converted to a variety of
other products such as paper boards, paper bags, folding cartons, and
corrugated and solid boards.

11.3 Physical Properties of Packaging Materials
During the manufacturing process and packaging fabrication, the
material is subjected to various physical treatments. A good knowl-
edge of physical properties of packaging materials is needed to design
and optimize physical processes of package manufacturing and fab-
rication. Physical properties of packaging materials are also impor-
tant in designing and selecting the appropriate type of packaging for
different food products to achieve desirable shelf life. The physical
properties in context of food packaging are classified in different cat-
egories: mechanical, thermal, optical, and mass transport properties.
These properties are very well defined, and appropriate scientific
methods are available for their measurement. The following section
describes relevant physical properties of packaging materials, refer-
ring to their importance in food packaging.