Ti-News n. 15 // January 2016 - page 16-17

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Technical Focus
Technical Focus
Plastic materials have been addressed as borrowed “petroleum”. This holds most specifically for
polypropylene and polyethylene, which provide exactly the same energy per weight as petroleum
does. This explains why these plastics require some protection against oxidation by ambient air, be
it during processing or when in use.
As we all know by experience we can handle any article made from polypropylene, including BOPP
film, safely as long as we don’t expose them to extreme heat conditions.
So it’s apparent that the reaction of polypropylene with the oxygen in air at ambient conditions is
a fairly slow process. This gives us the chance to interfere with any attack of the oxygen before any
damage is done. But we are better off if we do so quickly after the attack has started.
Our means to do so are so-called “stabilizers”.
The oxidation of polypropylene is a fairly complicated process which has been analyzed by one of the
pioneer companies in the field of stabilizers, Ciba-Geigy of Switzerland (now part of BASF), in great
detail. According to their findings the sequence of chemical reactions is best described in terms of 2
interconnected reaction cycles: Cycle I as in the drawing below depends of the recurring abstraction
of hydrogen atoms from polypropylene chains “R’-H” by so-called peroxy radicals “ROO°” generating
polymer chains bearing a chemical group known as hydroperoxide (“ROOH”) followed again by the
addition of oxygen regenerating ROO°. These hydroperoxide groups are crucial to initiate the even
more damaging Cycle II sequence of reactions which involve the formation of 2 radicals classified as
alkoxy and hydroxyl radicals RO° and °OH.
Both of these reaction cycles depend on a primary reaction, the
abstraction of a hydrogen atom by molecular oxygen, forming a
polymer chain radical “R°” which is replaced in both cycles with
similar radicals labeled R° or R’°.
This reaction fortunately is slower than any reactions of both the
cycles, but, for its rareness, not possible to check completely:
You cannot position a policeman at every street corner. So we have
to live with some degeneration of any plastic article, even if this
process takes 25 years or more as in polypropylene pipes for the
construction industry. The characteristic of these reaction cycles
is that the chemical groups that fuel, or catalyze these reactions
regenerate themselves.
Not that they are necessarily on the same polymer chain as the previous one, but the regenerated active
group has the same chemical structure and, for this, the same power to kick-off the next turn as the
previous.
What is even worse about Cycle II is that with each turn not only the initiator of the previous is replaced but
two secondary, similar powerful initiator group, RO° and °OH, are formed so that, if not checked in time, the
reaction sequence gets out of hand and degenerates into a true avalanche of a geometric growth pattern.
As mentioned before, there are 2 distinct conditions under which, polypropylene is exposed to oxidation:
- During processing the material is heated to a temperature of 200 °C / 390 °F or above.
- While in use the material is rarely exposed to temperature higher than ambient.
If you guessed that these require two different concepts of protection you are right. The current formula used
for the protection of polypropylene under both these conditions include 2 classes of chemical compounds
classified as phenols and phosphites.
For the processing of polypropylene, the phosphites are most important since they react much more readily
with the key element of the fast cycle than the phenols can do at these temperatures. A great advantage
of these phosphites is that the reaction product formed by the phosphite itself in the deactivation reaction
is colourless which has been highly supportive in the development of the plastic industry to what it is today.
In the early years, the yellow tinge of a plastic article indicated degradation and the near end of the articles
life-time which then came pretty fast since impurities in the polypropylene resin catalyzed its oxidation.
To overcome this perception phosphite stabilizers have been crucial. And before the introduction of
phosphite stabilizers, this perception has been supported from day one. For stabilization during use, the
phenol stabilizers are indispensible.
Being able to interfere with both Cycles, I and II they are already efficient at room temperature when
phosphites are too inert to do their job. With the high purity of modern polypropylene resins sufficient life-
times in pleasant appearance, meaning little discoloration, can be achieved for all kinds of applications, and
perfectly so for BoPP film in the full range of its use.
Stabilizers
in Polypropylene
1,2-3,4-5,6-7,8-9,10-11,12-13,14-15 18-19,20
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