Page 362 - Biosystems Engineering

Food Package Engineering 339

11.3.1 Mechanical Properties
The behavior of a material to mechanical forces is described in these
properties. Mechanical properties are important in selecting materi-
als and designs and producing packaging, optimization of transport,
and distribution.
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Density This is defined as mass per unit volume (kg/m ). There are
several standard methods available for measuring density of packag-
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ing materials. Densities of these materials vary from 800 to 8000 kg/m .
Density of materials is used to compare different physical attributes
of similar or dissimilar materials. The density of different foamed
plastics can indicate the amount of voids. Papers with different densi-
ties can indicate the level of fiber compaction.

Tensile Strength This is the maximum tensile stress that a material
can withstand before breaking. During tensile testing of the material,
three other parameters measured are elongation, yield strength, and
Young’s modulus. Elongation is defined as the percent of change in
original length of the material before it breaks. A large value of elon-
gation indicates that the material will absorb a large amount of energy
before breaking. Yield strength indicates the maximum stress the
material can take before it undergoes nonreversible distortion. Both
elongation and yield strength is important in the design of an unwind-
ing operation of the polymer film roll. Young’s modulus is defined as
the ratio of stress to strain over the elastic range (reversible deformation).
Young’s modulus is also useful for characterizing paper stiffness and
polymer-based packaging.

Burst Strength This is measured as the maximum pressure at which
the package fails. Pressure is applied at a right angle to the surface of
the package at a slowly increasing rate. The bursting test is common
for paper and polymer-based packaging. Metal cans and glass bottles
are also tested for their resistance for internal pressure.

Impact Strength This is a measure of package strength to withstand
a sudden (shock) load. This property is related to the material’s
toughness and the ability of the material to absorb applied energy.
The impact resistance of a package is measured using notched Izod,
or dart drop tests. In practice, packages may experience a sudden
load as a result of the package falling onto a rigid surface or knocking
against hard objects. The impact test is commonly performed for
glass, paper, and polymer-based packaging.

Tear Strength Paper and polymer-based packaging are frequently
subjected to tests that measure tear strength. The energy required to