Page 364 - Biosystems Engineering

Food Package Engineering 341

material indicates that is has better thermal insulation properties. In
general, amorphous solids have a larger specific heat than crystalline
solids. The specific heat of semicrystalline materials depends on the
amorphous and crystalline fraction in solids. Metals and glasses have
lower specific heat than plastics and cellulose materials.

Thermal Conductivity This indicates the ability of a material to trans-
port thermal energy within the solids by conducting the mode of heat
transfer. Thermal conductivity of solids depends on their atomic
structures and intermolecular bonds. Metals have larger thermal con-
ductivities than glass, plastic, or cellulose. Thermal conductivity of
plastic and paper-based packaging can be changed significantly by
incorporating the air of low thermal conductivity into the packaging.
Corrugated board and expanded polystyrene show better insulating
properties than solid board and plastics of the same thickness.

Thermal Diffusivity This is the ratio of thermal conductivity to the
product of density and specific heat. The rate of conduction heat
transfer in solids is proportional to their thermal diffusivities. Thermal
diffusivity of packaging material influences heating or cooling rates
during in-container thermal processing or cooling. Metal cans and
multilayer aluminum/plastic pouches show a very high rate of heat
transfer during thermal processing.

Coefficient of Thermal Expansion This expresses the relative change in
length for a temperature change at constant pressure. Most polymers
and alumina have a larger coefficient of thermal expansion than steel,
glass, and paper. The linear expansion in materials due to heat may
damage the container seal during thermal processing or cooling.

State/Phase Transition Temperature The melting point of crystalline
materials and glass transition temperatures for amorphous materials
are important during fabrication of packaging. The mechanical prop-
erties of materials are affected dramatically around their state/phase
transition temperatures. The amorphous and crystalline materials
above their transition temperatures can easily be formed into differ-
ent shapes. Amorphous packaging materials with very high transi-
tion temperatures behave like rigid solids at room temperature. The
molecular mobility in materials is restricted to their glassy state,
resulting in low diffusion of gases. Thermal and mechanical proper-
ties of materials are affected by their transition temperatures. 1,2

11.3.3 Optical Properties
Metal and paper-based packaging do not allow (visible) light to tran-
sit through them. For thermoplastic polymers and glass, a number of
optical properties of importance are transmittance, reflectance haze,
clarity, gloss, and refractive index. These properties are important for