Environmental Engineering Reference
In-Depth Information
Other environmentally benign plastic materials include those available directly from
plants, such as starches, starch-protein composites, triglycerides, and cellulosics
(www.greenplastics.com). It is important to understand that many such plant-based plastics
are presently competitive with petroleum-based materials on a cost-performance basis.
A clear example of this competitiveness is the case of Nylon-11 (polyamide-1 1), which
is made from castor oil extracted from castor beans. This tough and resilient material is
marketed as Rilsan by the Arkema Company (www.arkema-inc.com) and is widely used in
powder-coating applications. These demanding applications include protective coverings for
submerged pipelines and other industrial piping. In these uses, it is the superior properties of
the plastic, rather than its renewable nature, which has developed the market. While
biodegradable plastics are also available from petroleum-derived chemicals, this is not
accomplished using bioengineering techniques and so these materials fall outside the scope of
this document.
Polymer Nomenclature
Because the challenges associated with bioplastics development depend on the detailed
chemistry and physics peculiar to polymers, a set of useful terms is presented here for readers
less familiar with this specialized topic.
Polymer.
A high-molecular-weight organic compound with a repeating unit (a monomer)
that constitutes its structure. While typically represented by a long linear structure, different
chain architectures are possible by including branching points and other monomers. If two or
more monomers are present in the molecule it is referred to as a copolymer. Block
copolymers consist of long strings of the same monomer connected to other long stretches of
a different monomer whereas random copolymers have a random sequential arrangement of
the different monomers. Branched polymers consist of linear sections that diverge at a point
so that three or more strands emerge from the common branch point. In hyperbranched
polymers the divergence from one branch point leads to another and each of those branches
leads to another branch point and so on to create a highly branched or arborescent (tree like
branching) polymer.
Glass Transition Temperature (Tg).
The temperature at which the reversible change in
an amorphous polymer or in the amorphous regions of a semi-crystalline polymer change to
(or from) a hard and brittle glassy material to a soft, viscous, rubbery material. Hardness,
thermal expansibility, and specific heat all change abruptly at Tg, however, it is not a true
thermodynamic transition as it shows a dependence on cooling rate.
Melting Temperature (Tm).
The temperature where all crystalline structure is lost to
yield a liquid. Scientifically, it is incorrect to talk about the melting temperature of a fully
amorphous polymer, however, in practice a melt-flow temperature of 50 °C above the Tg is
sometimes used.
Polydispersity Index (PDI).
The ratio of weight averaged to number averaged molecular
weight. This index measures the breadth of the molecular weight distribution in a polymer
sample. If all polymer chains in a sample had exactly the same length, the PDI would be 1.0;
typical polymer samples have values from 1.5 to 30.0.
Tacticity.
Refers to the geometric arrangement of substituent groups (e.g., a methyl
group) along a polymer chain backbone. Syndiotactic indicates the substituents alternate
regularly on opposite sides, isotactic means the substituents are all on the same side, and
atactic means the order is random.
