Transmission Efficiency of plastic Films Part 1

TRANSMISSION EFFICIENCY OF PLASTIC FILMS
in outdoor use, especially in the tropics, or short-term, as in film extrusion where
conditions of high temperature obtain, albeit of relatively short duration.
The way in which oxidation may be slowed or inhibited can be seen by considering
the Oxidation processes themselves. With most polymers the initial effect of oxidation
is either depolymerisation or cross-linking. In the former instance, chain scission
occurs, with the formation of lower molecular weight oxygenated compounds. Among
these are acids, aldehydes, hydroxy acids, etc. The attack normally starts at double
bonds already present in the molecule or at tertiary carbon atoms (hence forming a
double bond). This sort of behavior is typical of polypropylene where oxidation can
eventually lead to the formation of very low molecular weight products, such as
butyric acid.
In the latter instance (cross-linking), the first products are higher molecular weight,
relatively infusible gels. These react further to give, eventually, lower molecular
weight compounds. This type of reaction is displayed by polyethylene. As a rule both
types of reaction are chain reactions, often involving hydroperoxy radicals. Hence,
the reactions can be slowed down or inhibited by compounds that interrupt the chain
reaction at some point.
The requirements of ideal antioxidants may be summarized as follows:
(a) They must be permanent.
(b) They must be capable of inhibiting oxidation under all conditions likely to be
encountered in storage, processing and service.
(c) They must be non-blooming.
(d) They must not discolor during processing or, later, when the product is exposed
to light.
(e) They must not adversely affect the characteristic properties of the base
polymer.
(f) They must be non-toxic and Odorless.
The oxidation processes are highly temperature sensitive. They may also be greatly
affected by other factors such as catalyst residues and UV radiation. Surface area
will also affect oxidative processes and this is an important factor with films. It is often
impracticable, therefore, to consider antioxidants in isolation. In practice, stabilization
systems are combinations designed to give optimum protection in relation to
oxidation, temperature and UV. In addition anti oxidants often display marked
synergism, so that many antioxidant systems comprise two or more compounds,
notably hindered phenols and organic sulphides. Hindered phenols (such as
butylated hydroxytoluene—BHT) act as free-radical scavengers or polymer chain
terminators. Other types of antioxidants, much as organo phosphites, act as
hydroperoxide decomposers.
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