Basic Technology of Plastics - Introduction to Polymeric Composites with Rice Hulls - page 16

The folded-chain alignment of molecules is called a lamellar (plate-like) structure. In
ordered polymers, chains are folded back on themselves to produce parallel chains
perpendicular to the face of the crystal. These crystalline structures within the polymers
help to scatter light. They give PE and polypropylene (PP) a milky appearance. Upon
normal processing, a crystalline polymer may begin to form spherulites as it cools.
Processing may help align or orient the molecular chains. Oriented alignment of
molecules on one axis may be desirable because this produces greater crystallinity and
higher tensile strength. Controlling the crystallisation process is a basic consideration
in determination of the physical properties of a polymer.
Materials that are amorphous (without form) have atoms or molecules that are non-
crystalline and without long-range order. An analogy of an amorphous polymer could
be a mass of cooked spaghetti. Polymers are amorphous in the molten state, and rapid
quenching may preserve most of this amorphous state. This method may prevent many
of the chains from realigning to form crystals. This method is used sometimes in the
production of PE and polyethylene terephthalate to improve transparency. Amorphous
polymers are usually less rigid than crystalline ones. Polymers are transparent if
most crystallinity can be prevented. LDPE will be more transparent if crystallinity is
reduced. HDPE is semi-crystalline and has a high melting point.
Many components are needed to form a plastic, but a polymer are is basic material
from which a plastic is formed. Fillers, solvents, plasticisers, stabilisers, colourants
and additives influence many of the characteristics of plastics. A resin is the polymeric
material that helps impart many of the physical characteristics of solid plastics.
It is the molecular arrangement of the polymer resin that determines whether a
plastic is thermoplastic or thermosetting. Thermoplastics increase in plasticity with
temperature; they become soft with heating and solid if cooled to room temperature.
Thermoplastics can be formed readily into any shape because weak bonding forces
allow slippage between molecules, whereas the molecules themselves are held
by the stronger covalent bonds. Thermoplastics can be re-used, but there is a limit
to the number of times recycling can be done because repeated processing causes
some the additives to be lost and thus causes degradation.
Thermosetting plastics are polymeric materials with structural frameworks that do
not allow deformation or slip to occur between molecular chains. They are composed
of strong, primary covalent bonds and may be thought of as one large molecule.
In thermosetting materials, heat is commonly used to cause a chemical reaction
(polymerisation) resulting in crosslinks between chains. In a low-molecular-mass state,
heat and pressure are commonly used to cause the thermosetting material to flow
into a mould cavity. Once solidified, these materials cannot be reshaped or formed
again by the application of heat. These materials have a permanent 'set' once they