Page 548 - Carrahers_Polymer_Chemistry,_Eighth_Edition

Additives 511

Experimental investigations and assessments show that the average person takes in about 2
grams a year of external plasticizers. Most of this is from traces of DOA migrating from food pack-
aging. The so-called no observed effect level (NOEL) for DOA in rodents is about 40 mg/kg of body
weight per day. Extrapolation for a person equates to 1,000 g of plasticizer for a “safety factor” of
about 500. Even so, increased efforts to evaluate the safety of plasticizers continue.
Plasticizers extend the lower temperature range for use of materials since they discourage poly-
mer chain associative behavior and encourage segmental flexibility increasing the rotational free-

dom effectively decreasing the material's typical T .
g
When present in small amounts, plasticizers can act as antiplasticizers increasing the hardness
and decreasing the elongation of the material partly due to their ability to fill voids. Ineffi cient plas-

ticizers require larger amounts of plasticizers to overcome the initial antiplasticization. However,
good plasticizers such as DOP change from being antiplasticizers to plasticizers when less than 10%
of the plasticizer is added to PVC.
External plasticizers are not permanent. Plasticizer molecules associate with one another even-
tually creating “preferred” migration routes to the material’s surface where the plasticizer is rubbed
or washed away. The preferential association of plasticizers also leaves some sites less fl exible and
creates variations in the material’s stress–strain and expansion–contraction behaviors.
As noted elsewhere in this text, the health issues related to plasticizers is an issue particularly
with respect to their use in materials that come in contact with our food. While the leachability of
most plasticizers is small, it is real and ppm to ppt amounts of some plasticizers have been found to
leach to the surfaces of food packaging materials. Thus, efforts are aimed at lowering to near zero
the migration of plasticizers to surfaces as well as using safe plasticizers so that even when migra-
tion occurs, the health effects are near zero.

15.2 ANTIOXIDANTS
Antioxidants retard oxidative degradation. Heat, mechanical shear, and ultraviolet (UV) radiation
can be responsible for the formation of free radicals, which in turn, can act to shorten polymer
chains and increase cross-linking, both leading to deterioration in material properties. Free radi-
cal production often begins a chain reaction. Primary antioxidants donate active hydrogen atoms
to free-radical sites thereby quenching or stopping the chain reaction. Secondary antioxidants or
synergists act to decompose free radicals to more stable products.
Polymers such as polypropylene (PP) are not usable outdoors without appropriate stabilizers
because of the presence of readily removable hydrogen atoms on the tertiary carbon atoms. PP and
many other polymers (R–H) are attacked during processing or outdoor use in the absence of stabi-
lizers because of chain degradation reactions as shown in the following:
By initiation:

R−H R + H
(15.6)
Polymer Free radicals
By propagation:

R + O 2 ROO
(15.7)
Free Oxygen Peroxy free
radical radical

ROO + R−H ROOH + R
(15.8)
Peroxy free Polymer Dead polymer Free
radical radical

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