Chemistry Reference
In-Depth Information
In the polymerization to give PET, any diethylene glycol that is formed will
react, just as ethylene glycol does, and incorporate into the polymer chain.
PET is a crystalline polymer and its melting point of about 260
∘
C determines
its upper use temperature. If a significant amount of diethylene glycol is
formed, it will incorporate into the polyester and lower the melting point.
It will also decrease the crystallinity of the PET. Therefore, instead of using
sulfuric acid the acid catalyst is typically a Lewis acid that enables the ester-
ification reaction to preferentially occur. Antimony oxide is commonly used.
Polyesters are also made by transesterification reactions. Terephthalic acid
reacts with methanol to give dimethyl terephthalate (DMT) which then reacts
with ethylene glycol to give PET. Again a Lewis acid is used as a transesterifi-
cation catalyst. Common catalysts are tetraalkyl titanates or antimony oxide.
O
O
H
H
CH
3
C
O
O
C
+ 2x CH
3
OH
+
OH
H
H
O
O
H
3
C
OH
O
O
x
DMT
Although conversion of terephthalic acid to DMT adds an extra step, there
are some advantages that make it attractive. Terephthalic acid is a solid that
sublimes at 402
∘
C and must be handled as a solid. DMT melts at 141
∘
Cand
can be distilled. Therefore, DMT can be purified by distillation. This makes
high purity PET possible. Also, DMT can be shipped, stored, and transferred
as a liquid. It is much easier to pump a liquid than to transfer a solid. Solids
take up more room (solid terephthalic acid density is much less than molten
DMT) and therefore less can be shipped in a certain truck volume and larger
storage tanks are needed. It is more difficult to transfer a solid from storage
to a reactor.
The largest use for PET is for fibers. Much clothing and carpeting is made
from PET fibers. Another large use for PET is plastic water bottles and plastic
soda bottles. There are many other uses for PET; film is another major appli-
cation. North American polyethylene terephthalate capacity is estimated to
be 10.3 billion pounds [12].
Coca-Cola uses PET that is made from bio-based ethylene glycol as a basis
for its PlantBottle
®
, which they use for a portion of their soda and water
bottles. The ethylene glycol is made by dehydration of plant-derived ethanol
to make ethylene, which is then converted to ethylene oxide and then ethylene
glycol. There is some ongoing research to make bio-based terephthalic acid;
another route is to substitute the terephthalic acid with furan dicarboxylic
acid, which can be derived from sugars. Avantium is starting a 40 metric ton
per year pilot plant to make furan dicarboxylic acid and is in the planning
stage for a 50,000 metric ton plant [13].
Search WWH ::
Custom Search