IV Extract uit: 'Melinex' Polyester Film
Nature and Uses
Introduction
'Melinex' is the registered trade mark of polyethylene terephthalate film manufac
tured in Great Britain by the Plastics Division of Imperial Chemical Industries
Limited.
For a little over a decade the only form in which polyethylene terephtalate poly
mer has been fabricated in the United Kingdom is that of fibre, sold under the
registered trade mark 'Terylene.' More recently, however, attention has been direc
ted to the polymer's film-forming characteristics, and a considerable amount of
work has now been carried out on the manufacture of 'Melinex' polyester film.
'Melinex' film is tough, transparent and flexible, with a high surface gloss, and
has outstandig mechanical strength over a wide range of temperatures. The elec
trical properties of volume resistivity and dielectric strength are excellent, and
because of its low water absorption the film retains these properties at high humi
dities. Although 'Melinex' film does not fall within the class of low-loss dielectrics,
its combination of properties makes it extremely attractive to the electrical industry
for use in capacitors and insulation, especially at high temperatures. 'Melinex' film
has very low gas and odour permeabilities and a low water vapour permeability, is
impermeable to fats and oils, and has good chemical resistance; this additional
combination of properties makes the film of great interest to the packaging industry.
Indeed, the film is so versatile that there are few industries that will not find some
application for 'Melinex' film as experience and knowledge of its capabilities be
come more widespread.
History and Production
Polyethylene terephthalate was discovered in 1941 by Whinfield and Dickson in
the U.K., following work on the general problem of the relation between crystal-
linity and molecular structure of high polymers. It is a polyester formed by the
combination of ethylene glycol and terephthalic acid with the elimination of water:
In practice it is more convenient to use the dimethyl ester of terephthalic acid and
to eliminate methanol by ester interchange with ethylene glycol.
The polymer can exist in various physical forms: it may be amorphous or it may
have varyving degrees of crystallinity, depending on the thermal treatment it has
received. It has a sharp melting point of about 265°C. (509°F.), and if the polymer
is heated above this temperature and then quenched rapidly, X-ray examination of
the quenched material shows virtually no crystallinity. If this amorphous material
is heated it is stable up to the second-order transition temperature of about 80°C.
(176°F.), but at higher temperatures crystallisation commences, the rate and
amount increasing with temperature until a maximum is achieved at about 170°C.
(338°F.).
Because of this amorphous/crystalline behaviour the manufacturing process for
'Melinex' film is more complicated than for, say, polythene film, and gives rise to
different forms of film, all of which are likely to be of importance to certain indu
stries. It is therefore of interest to consider briefly the manufacturing process.
This may be divided into three stages.
First, the polymer is melted and extruded through a die to produce a molten film
that is quenched rapidly. This treatment gives an amorphous material which is
reasonably strong, but which is unstable and becomes opaque and brittle if heated
above 80°C. (176°F.) for more than about one minute.
In the second stage of manufacture the film is 'plane oriented' by heating it to
above 80°C. (176°F.) and drawing it equally in two directions at right angles.
Drawing improves the mechanical properties of the film and it is no longer brittle.
If subsequently heated above the drawing temperature the film, if free to do so,
will contract considerably.
The third stage, therefore, consists of holding the film so that it cannot contract
and heating it to about 200°C. (392°F.). A considerable amount of crystallisation
occurs without appreciably affecting the orientation, and the film becomes substan
tially stable dimensionally up to the crystallising temperature. It should be noted
that drawn, crystallised film is highly transparent, whereas undrawn, crystallised
film has a milky appearance.
As normally supplied, 'Melinex' film has passed through all three stages of manu
facture and is therefore a plane-oriented crystallised film. It can be supplied, howe
ver, in experimental quantities with first or second stage processing only, and these
films are likely to be of interest in certain industries. For example, first stage film,
which is amorphous, may be vacuum or pressure formed, although the amount of
draw in all directions and the thermal conditions under which forming takes place
must be carefully controlled if a tendency for splitting and opacity, because of
premature crystallisation, is to be avoided.
It should be noted that, as well as 'Melinex' polyester film, other chemically
similar films are available under various trade marks in different parts of the
world.
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