1. Field of the Invention
This invention relates to electrical devices compris- It has now been discovered that a PTC conductive
ing PTC conductive polymers. polymer based on a crystalline polymer has substan
2. Introduction to the Invention tially improved electrical properties, in particular when Conductive polymer compositions exhibiting PTC 10 subjected to high voltage stress, if it is cross-linked in
behavior, and electrical devices comprising them, are two steps and is heated between the cross-linking steps,
well known. Particularly useful devices comprising to a temperature above the temperature at which the
PTC conductive. Polymers are self-regulating heaters crystals begin to melt (referred to herein as T/), and
and circuit protection devices. Self-regulating heaters preferably above the temperature at which melting of
are hot and have relatively high resistance under nor- 15 tne crystals is complete (referred to herein as TM)- For
mal operating conditions. Circuit protection devices are example, if two identical circuit protection devices are
relatively cold and have a relatively low resistance irradiated to the same total dose, one in two steps with
under normal operating conditions, but are "tripped", n0 intermediate heat-treatment step, and the other in
i.e converted into a high resistance state, when a fault tWQ st with an intermediate heat-treatment above
condition, e.g., excessive current or temperature, oc- 20 T^ the latte roduct has substantiall better tolerance
curs. When the device is trippedI by-excessive current tQ e(J ^ > „ a{ Uh yol ( at m ,
he current passing through the PTC element causes it AC ^ ^ pT * « ag
to self-heat to an elevated temperature at which it is in , . „y. . „ T. . it_ _? , x it_
a high resistance state. Circuit protection devices and dunn« the ***** P^l"8- l} 18 th«OI?zed that the
PTC conductive polymer compositions for use in them, 25 new process results in a different cross-lmked structure
are described for example in U.S. Pat. Nos. 4,237,411, fch that the resistivity/temperature curve of the con
4,238,812, 4,255,698, 4,315,237, 4,317,027, 4,329,726, ductive P°lymer is changed so that at least at some
4,352,083, 4,413,301, 4,450,496, 4,475,138, 4,481,498, elevated resistances, a particular device resistance is
and 4,562,313; and in copending, commonly assignedd reached at a lower temperature.
U.S. application Ser. Nos. 141,989 and 628,945. Other 30 It has also been discovered that a PTC conductive
applications which are related to this application are the polymer device has improved properties, for example a
copending, commonly assigned applications filed con- broader hot line and/or a more rapid response, if it is
temporaneously with this application by Deep et al, Ser. cross-linked in such a way that a center section between
No. 711,909, by Carlomagno Ser. No. 711,790, by Ra- the electrodes absorbs a radiation dose which is at least
tell, Ser. No. 711,907, and by Ratell, Ser. No. 711,908. 35 1.5 times the radiation dose absorbed by portions of the
The disclosure of each of these patents and prior filed PTC element adjacent the electrodes,
pending applications is incorporated herein by refer- Particularly useful results are obtained when these
ence. two discoveries are combined. For example, in this way
In many devices, and especially in circuit protection it is possible to produce circuit protection devices
devices, it is desirable or necessary for the PTC conduc- 40 which will withstand repeated tripping at 1 amp and
tive polymer to be cross-linked, preferably by means of 600 volts AC and which, for a particular resistance, will
radiation. The effect of the cross-Unking depends on, trip more rapidly than a similar device in which the
among other thmgs, the polymer and the conditions whole of the pjc element is irradiated in both steps,
during the cross-linking step, in particular the extent of In its first aavecU this invention provides a process for
the cross-linking, as as discussed for example in copend- 45 the preparation of m electrical device which comprises
?M> £°oT°^y 1aBM«n«i aPP«cation Ser No. (1) a dement composed of a cross.iinked con.
468,768, the disclosure of which is incorporated herein ductive j c0 ition which exhibits PTC
by reference. When a conductive polymer element is . , „„ J „,u;„u „~ „■ „ „ i •„ „„
.J ,. . lf ... , f' . , behavior and which comprises a polymeric compo
urradiated, the radiation dose absorbed by a particular „ . „„„,„,•■ „ „ „„,o;„ii;„» Tma
. r V , ^ . . ... , .1 ... „„ nent comprising a crystalline polymer and, dispart of the element m a given time depends upon its 50 , . *\. , . e .. , .
. , - e... . j Jli-l. persed in the polymeric component, a particulate
distance from the surface of the element exposed to the A *' rn H
source, and the intensity, energy and type of the radia- TM°? , ?V a5! u , . • ,,
tion. For a relatively thin element and a highly penetrat- <2> tw° eilc*r°f68 which are electncally connected
ing source (e.g. a Cobalt 60 source), the variation of t0 the ^ dement and which are connectable to
dose with thickness is negligible. However, when using 55 "ourcf °? el^"cal P°wer t0 cause current to Pas
an electron beam, the variation in dose with thickness through the PTC element,
can be substantial; this variation can be offset by expos- whlch Process comprises the steps of:
ing the element to radiation from different directions, (a> subjecting at least part of the PTC element to a
e.g. by traversing the element past the source twice, ^rst cross-linking step,
irradiating it first on one side and then on the other. 60 (b) heating at least part of the cross-linked PTC ele
Depending upon the energy of the beam and the thick- ment to a temperature above T/, where T/ is the
ness of the element (which can of course vary, depend- temperature at which the conductive polymer
ing upon its shape), the radiation dose can be higher at starts to melt,
the surfaces exposed to radiation than at the middle, or (c) cooling the cross-linked and heated PTC element
substantially uniform across the thickness of the ele- 65 to recrystallize the polymer; and
ment, or higher at the middle than at the surfaces ex- (d) subjecting at least part of the cross-linked, heated
posed to radiation. In addition, the radiation dose near and cooled PTC element to a second step to effect
the surface exposed to the radiation can be less than further cross-linking thereof.
