Chemistry Reference
In-Depth Information
order to avoid the necessity of introducing corrections to molecular-weight distribution the
tests are carried out on thin films (3-5 nm) with optical density not higher than 0,35. Exposure
doses must be low causing no more than one break counting one initial macromolecule [246].
The rate of any photoreaction equals v=ФI
abs
, where Ф - quantum yield; I
abs
- amount of
light, absorved in unit time.
Thus, the most important factors, on which PETP destruction depends, are intensity of
irradiation, wave length and presence of oxygen. In order for destruction to proceed according
to mechanism of direct photolysis PETP must absorb the light with wave-length not higher
than 340 nm, moreover the process of destruction must proceed in the atmosphere of inert gas
or in vacuum. In this case the main effects are joints between polymer chains.
Photoxidative destruction of PETP will proceed in the presence of oxygen. Breaks of
macromolecules chains will take place in polymer, but lateral joints between chains are not
observed. Two processes proceed during photooxidation at the same time: first - radical-
chain, second - with singlet oxygen taking part in it.
3.2.
M
ETHODS OF
PETP
M
ODIFICATION
Modification of physical and chemical structure of polyster fibres allows to broaden
greatly the assortment of these fibres, to give new valuable properties, which, in some cases,
are not characteristic for the fibre from homopolymer.
There may be different directions of modifying action: thermal stabilization, light
stabilization, plasticization, dyeing.
There are two methods for stabilization of polymers:
(1)
introduction of special additives-stabilizers;
(2)
modification by physicl and chemical methods.
The example of the second method is: giving fire-retardant properties to PETP -
antipyrine is introduced by the way of additive mixing with polymer [247] plasma
modification is used with the aim of improving adhesion [248].
The first method is more perspective, as stabilizers may inhibit destruction reaction; may
directly influence the mechanism of destruction with the purpose of decreasing undesirable
products yield and so on. It is supposed that stabilizers may act by means of: 1) blocking of
active centres (weak bonds); 2) filtration of ultra-violet radiation; 3) breaking of peroxides; 4)
interaction with free radicals; 5) suppression of excited states.
Connection of modifier structure with the effect of its action and also interconnection of
polymer structure with the structure of either additive are not finally revealed up to now. For
many modifiers mechanism of their action is not known at all, though their efficiency is
known.
While selecting modifier it is necessary to consider compatibility of the additive with
polymer, volatility and extraction, dyeability, toxicity, odour, economical efficiency, effect on
technological mode of processing, service properties of polymer materials.
One of the methods of introducing additives into PETP is modification at the stage of
synthesis. To increase strength and resistance to thermodestruction of PETP filament it is
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