Note: Descriptions are shown in the official language in which they were submitted.
I 49/ 201
.
lZ(~507
n~ ~ ntiO . relat~ to elec~ic~l de~lc~ in whict~ an
.
a~pris~ o~Anlc E~lym rs h~vir~ d~rs~ ~r~in ~ ~n~ly divid~d
S ~ ill, ~ a~m blac:3c or a p4rtia~lAt ~etal.
5~ ~uch ~ ath~bit ~d P~C (Po~i~ve 5~~
C~icient) ~vior. m~ ~y ~ h~ b~ used in the p~t
to de~ P~C b~vior ~ variabl- ad o~ ~r-ci~. ~n
c ~ ositicn~ d-note a o~mpo~it~ ving at t~ant on
t ~ ture rang- (herQina~t~r c211~d a critic~l rang~) wh~ch i~
~ithin th- llmit~ o~ -lOO-C an~-ak~ut 250-Cs at th ~ ginning of which
the oo~pw ition haJ a resi~tivity.bolo~ about 105 ohQ. c~.~ and ~n
which the ccnpw ltion ha~ an ~4 valu- o~ at l~t 2.S ~r an ~0O v~lu~
of at le~st 10 ~And p~ferably both), and pr~f~rably ha an ~ 0 valu
of at least 6, where ~4 i3 th~ rat~o of the red~tiviti~ at the ~nd
an~ the begi ~ of a 14-C rang~, ~0O is th~ rat~o of th~
re~i~tivitics at th 2nd ~nd the bQginnlng of a lOO-C r~nq-, and ~0 i~
the ratio of the r L~tlviti~ at th~ end and the beginnlng of a 30-C
range. The tcr~ ~PTC el~mentR i~ us~d herein b~ denotc an element
ccmpo~d of a PrC oompo ition as definod abov-. A plot of the lcg of
the r i~tance of a PTC element, meaJur-d between two electrode~ Ln
oontact with tha element, against tcmptratux~, will often, t ~ h by no
me~n~ invarlably, ~ho~ ~ aharp ch4ng- in ~16Qe w er a pa~t o~ th
cr~tical t6np~raburo r~ng~, ~n~ In such ca~e~, th~ ter~ ~witch~ng
trmperab~r~ (usually abbr-viat~d to T5) i~ u~d her~in to denot- the
to~peratur- at th~ ~nO~rs ctlon point o~ exten~on~ of the
sub~tantially ~traight portion~ of ~uch a plot whlch li- elther ~ide of
th~ portion s~wirq th- sharp chang~ . m~ Prc c~mFo~ition in
~ a PrC de~t i~ de~crib~d h~in ~ h~ u~ful T~. me
prefera~ly b~n ~ and 17S-C., ~.g. 50-C to 120-C.
12(~6S~7
Ca~uctive p~lymer c~T~sitions, especially PrC ç~sitions,
are u~eful in elec~lc~l d~ccs ~n whi~ the o:snposition i~ in contact
with an elec~cde, u~uaLly of met2Llr Device~ of this ~cind are usually
manufactl~red by metldq ~pr~3~ng at~l~ or mou~d~ng the m~ltal
S polymer ~ sition ~round or ag~ ~st tl~ electr~de or el ~ des. ~n
the known methods, the electrode it not he~te~ prlor to oontict with
the polymer w utusition or i~ heated only to a l~mitsd extent, for
examçlc t~ a temperature well below the melting point of thc
.~u~u~ition, for example not re than 65-C as in conventional wire-
ooatinq tech~iques. ~Temperaeures ~.o in C throughout thi5
specifica ion.~ Wkll known exAmçle~ o~ such t~vices are n exl~le st-ip
heater~ which ~I~Li9~ a gener~lly ribbonrsh~ped o~re of the conductive
polymer compo~itlon, a pair o~ lonqitLdinally extendLng electrcdcs,
gener~lly of stranded wire, mboddod in the o~re near the edges
thereof, and an outer layer of a protective and msl~latlng ccmposition.
Particularly useful heater~ are those in which th~ composition e~hibits
PrC behavior, and which are therefore self-regulating. In the
preparation of such heaters in whlch the comFosition contains less than
15% of carbon black, the prior art has taught that it is necessary, m
order to obtain a sufficiently lcw resistivity, to anneal the heater
for an extended period such that
ZL + 5 logl~ R< 45
where L i5 the percent hy weight of carbon and R is the resistivity in
ohm.c~. at roon temperature.
A disadvantage which a~rlse~ with devices o~mprising an
d ectrcde and a conductiv~ polymer oomposition in contact wnth the
electrodc, and in particular with strip heater~, is that the longer
they are in ser~rice, the bigh is t!~eir resistance and t~e lo~e- is
their E~wer out~t, partlcularly ~hen ~hey are s~ject b~ thennal
cycling.
lZ(~6S~t~
It is ~n t~at var~ au, ~n device ~ device, of the
~ntact resi~t~nce betw~ electr~e~ arr3 carb~lack-filled rub~
lt an obst~cle ~ c3~ari90n of tl e electrlcal characteristi~ of such
devices ar~ e acc~at~ measureneslt of the re~i tiYity of such
S rubbers, par~:ioularly at high re~ vitie~ and low voltages; ar~ it
has be~ su~ted that the saDe i~ ~ue of oth~r conductive polymer
amE~itions. Variou~ method~ h~ve bean su~qested for re~ucir~ the
~ntact resi tance bet~ c~bl~ck-fi1led n~ers and test
ele~e~ plac~d in ccntaot therewith.. m~ preferr~ methcsd is
vulcan~s~ the rubb~ while it i~ in contact with a bra~ el~ode.
Otl~ ~etlx~ds includ~ ~pcr-~lat~r~, vacw~ating ~nth ~old, ar~ the
use of coLtoidal solutions of graphite betwe~ the electrcde arx3 the
test plece. Ebr de~ s, r~ference s ~ ld bo made t~ ~ upter 2 of
~Conductive Rubber3 and Pla~tics~ by R ~. Nor~an, published by Applied.
15 Science P~blisher~ ~1970), from which it will be clear that the factor~
which govern the size of such 03ntact re~istance arc not well
understood.
We have now disow ered ~hat the less is the initial contact
resist~nce ~etween an electrode and a conductive polyme comFosition,
the smaller i~ the incrc~s~ in total resistance with tLme. ~e have
also disc~vered that by placiny or m~intaining an elect cde and a
polymer composition in oontaot wit~ each other while both are at a
temperature above the melt~nq point of the ~ vsition~ the oontact
re~istance between the~ is reduc~d. The teDm ~melting po m t of the
oomposition" is used hein to denote the ~rature at which the
oomFo~ition begin~ t~ m d t. The time for which the electrcde and the
03mpo~ition ne~d be in oontact with each other, ~hile e~ch is at a
temperature above the meltlng point of the composition, in orde- to
achieve the desired r ult, i~ quite short. TLmes in exces~ of five
lZ(~6S(~7
m m utes do not result in any substantial fucther ceduction of cont~ct
rc~i~tance, and often times les~ than 1 m m ute are quite a~equate and
are therefore preferre~. Thus the ~reatment time is of a quite
diff ent order from ~hae requl~ed by the known annealing treatments bo
deccease the resastlvity of the ccmçosition, as described for exæmple
in ~.S. Patents No~. 3,823,217 and 3,914,363.
In one aspect, therefore, the inv~ntion prcvide~ a ~ ess
~or t~e prqparation or modification o~a device ~L~ ng an elect ode
and a con~uctiv~ polymcr oomp w ition i~ oontact wi~h the clectrode,
0 whic~ xoe3~ CampriX3 contaCt~ng~ or mdintain~ng conta~t bet~een, the
~uctive p~lymcr ocmposltlon and the d ectrcde whlle the oonduc~ive
polymer oomposition i3 at a te~Feraturc ~$p) ab~ve it~ melting polr.t
(Tm) and the electrode is at a t~nperature ~Te~ abcve the m d ting poin~
of the conductive ccn~oo~ition, Tp and Te beinq the same or different,
for a time which is sufficient to reduce the oantac~ resis nce between
the electrode and the conductive polymer composition but whidh is not
sufficient-substanti~lly to reduce the resistivity of the oonductive
Folymer. Preferably both Tpa~d Te are at leas~ 20-C, especially at
least 55-C, above Tm~ It is often preferable that both Tp and Te
2~ should be above the Ring-and-Ball softening t~mpera e of the polymer.
ocmposition.
Preferably the conductive polymer conposi~ion is subjected
to pre5sure to assist m bringing it into c!ose ~onfo~ity with the electrode.
The p¢essure i~ gen ally at lea~t 14 kg/c~2, prefcrably at least 2l.
kg/c~2, for example 21 to 200 hg/cm2, especially at lea~t 35 hg/cm2,
e.g. 3S-70 kg/cm2.
-. lZ~6S~7
We ~ave als~ foun~ that the contact resistance c~n be
~rrelated ~th the force ne~ded to pull the elecl.L.~le fmn the Folymer
c~ltial. Acc~rd~r~ly the ~nv~elctl further prwide~ ~ deviY
ccn~rising N~ ele~e ~ contac~ ~ith a ~nd~elve polyme~
~ltion, especi~lly a ~tr~ded w~re electrode ~ dded.~n a
ccx~ductive polysner ~sieio~n, the pull s~cren3th (P~ of ehe elec~ode
fr~m the de,lice beir~ equal t~ at least 1.4 time~ PO, where PO i3 the
p~lll stre~th of an ident~c~l elec~cde fml~ a devi~e whieh .~ises
~n idultical elec~de in clt~ct. with an identical conductive p~lymer
0 c~mposi~ and which ha~ pared b~ a pro~ess whi~:h con~ist~ of
contacting the el~c~ode, while it is at a temperature not greater than
24e~, w~th the x l~en oonductive polymer oomçosition, an~ allownng t~e
polymer coDposition to ~col in contact ~ith the electrode. The pull
strengths P and PO are detesmined ~t 21C as describçd in detail below.
A 5.1 cm long sæmple of thc heater strip (or other device),
contaLninq a-straight 5.1 cm length of the electrcd~s, i5 cut off. At
one end of the samçle, 2~5 cm of the electrcde is stzipped bare of
polym OE . rhe bared elect-cde is passed dcwnwardly t.~rough a hole
slightly larger than the electrcde in a rigid metal plate fixed in t~e
horiDontal plane. The end of the bared electrode is fi~ly clamFed in
a mcvable clamp below the plate, and the other end of the sample is
lightly cla~ç~d above the plate, so that the electrcde is vertic~l.
m e movable cl~mp i~ then moved vertically dcwnwards at a speed of 5.1
cm~min~, and the pe~k force needed to pull th; conductor ~ut of ~he
sample is me~sured.
.. _ _ .. _ _ .. . . _ . . _ . _ . _ ~ . .
lZ-~S~7
Wk h~ve also found that ~or strip heaters, curren ~y the most
widely ~lCAd d~vic~s m which current is passed through conductive
polymer oompo~it~on~, e~e oontact resi-~tance can be o~rrelated with the
linearity ratio, a qu~nt$ty which can re~dily be ~easured as described
bel~w. Accordingly th~ 1nventlon ~ur~her provides a strip heatcr
oomprising:
(1) an elongatc core o a conductive poly~er comçosition
which exhibits ~TC beh~lor, whlch comprise-~ car~on blac*, and in
which, if the oontent (L) of carboQ black Ln percent by weight is less.
~han lS, L and the resi~t$vity R o~ the oompo3ition in oh~. om are such
that
2~ ~ 5 log10 ~ > 45
~2) at least tw~ longit~dinally extending electrcoes
embedded Ln said composition par~llel to e~ch other: and
IS `(3) an out~r layer o~ a protectlve and msulatmg
comFo~ition;
the average line~rity ratio (and prefcrably the linearity ratio at 211
points~ betwe~n ~ny pair o~ elect mde~ being at ~ost 1.2, pre~erably at
most 1.15, especially at mcst 1.10. m e l m e~rity ratio of a strip
heater is defined as
Resistance at 30 MV.
Resistance at 100 V.
the resiQtances be mg m~swr~d at 21C between tw~ electrodes which are
contacted by prcbe~ push~d thro~yh the outer jacket and the oonductive
polymeric sore oS the strip heater. The o~ntact re-~istance i~
neql~gible at 100 V., ~o that the closcr the l~n ity ratio i3 bo 1,
the lower the contact re~istance.
The LnVention i~ useful with any txpe of electIode, for
exaTçle~plates, strip~ or wires, but particularly so with electrcdes
haYiny Dn irregular surfac~, e.g. str nded wire electrcdes as
conventionally used ln ~trip hcaters, br~ided wire electrode~ (for
examçle a descr~bed ~n Ger~an Offenl q ungschrift No. 2,635,000.5)
~2~6S07
and expar~able elect~de5 as describ~ in Gerrran Off~legungs*~ift No.
2,655,543.1. Preferred ~tranded wire~ are silver-)at~d and nic~
~ated copper w~res, wh1~h are 1e~Q su~tible to difficultie5, such
as melt~ or oxidatiotn, than t~coats~ or unc~ted copper wire~,
S t~uqh the latter can ~e usad wit~ut difficulty pr~vid~ng the
t~at~es ~r~l~d are not too high.
The c~uctive polyn~ c~psitioru ~sed in thi~ ~nves~tion
gener~lly c:on~n car~on black a~ the conductive filler~ e.g. in zn~t
gre~ter or le53 than 15% by weight, f~r example greater t~an 17% or 20%
by wcight. In many caxs, lt ~ prderr~d that the ca~sitionJ s~ld
exhibit ~C behavior. The resistivity of the c~osition i5 generally
le~s than 50,000 oh~ at 21-C, for ex ~ le 100 to 50,000 ohm.~n. Fbr
strip h~ters designed bo b~ powered by A.C. of 115 ~olts or more, the
composition generally ha~ a resis~vity o~ 2,000 to 50,000 ohm.c~, e.g.
2,000 to 40,000 ohm.cm. 5he oomposition 1~ pr~ferably thesmoplastic.
~owever, it may be ligh~ly cro59-linked, or be in the process of being
cross,linked, provided that it is sufficiently fluid under the
contacting condition to conform closely to the electrode surface.
The polymer is preferably a cry-st lline polymer.
The strip heater~ with which the invention is concerned
generally have two electrcde~ separat~d by a distance of O.lS to 1 c~,
:-: but gre~ter separations, e.g. up to 2.5 cm. or even more, can be used.
m e o3re of oonductive polymer can be of c~nventional shape, but
preferably it has a cros~-sectl~n which is not re than 3 times,
especi lly nct re than 1.5 times, e.g. not more than 1.1 times, its
smallest dimension, especially a round cross-sect~on.
_7_
lZ~65~7
In one pre~erred embcdiment of the ~nveneion, the electrcde
and the polymer composition are hea~ed ~epara~ely ~efore be mg
contacted. In this ol~Jlment, it i~ p~eferred that the coTposition
should be melt-e~truded cver the electrcde, e.g~ by ~xtrusion around a
S pa~r of spaced-apart wire electrodes using a cross-head die. m e
el ~ e i3 pre-heated to a t2mperature (Te) which may be greater or
les~ than the melt temperab~re of the polymer .~4usition (Tp) bu~ i~
gen ally more than ~Tp-55) and prefer~bly more than (Tp-30). Tp will
norm~lly be substanti~lly ab w e the meltlng point of the oomposi~on,
tO ~or examçle 30 to 80C abcv~. Of o~urse, nelther the elec ~ ~de nor the
ocmp~ition sh2uld b~ he~tod to a t3mp~rature at which ~t undergocs
substanti 1 ox~dation or oth degradation.
In another embodiment of the m vention, the coTposition is
shaped in contact w~th the elec W e (wnthout pre-heating the
electrode) and the elec~rode and the composition are then heated, while
in oontac~ wnth each other, to a t~mperature ab~ve the melting ~OLnt of
the composition. Care is needed to ensure a useful reduction in the
contact resistance by this method. The optimum conditions wlll
depend upon the electrode and the ~usitiGn~ but inc~eased ti~e,
t~mperablre and pressure help to ac~ieve the desired result. me
pressure may be applled for examç~e in a press or by means of nip
roller~ is embodiment o~ tbe ~nvention is particularly use~ul when
tbc need for, or deeir~bility of, an anneAling treatment does not arise
at all, for exam~le, w~hen the ocmposition ha~ a c~rbon blac~ oontent
gre~ter than 1~% by weight, e.g. greater than 17% or 20% by weight, or
when only a limlted anne ling treatment is carried out, such that at
the end of the annealing tbe o~ntent of c~rbon black tL) and the
resis~ivity (R) are sucb that 2L + 5 log10 > 45.
one w~y of heatlng the eleotrcde and the ccmposition
surr~unding it is to pas~ a high current ~hrough the electrcde and thus
produce tbe desired he~t by res~stance heating of thc electrcde.
:12(~65~7
~n another ~mbodlment Of the invention, the conductive
polymer 03mposition is initialiy in the fonm of a shaped article, e.g.
one or xre pill~ or pellet~, whlch ha~ not been 5hyped in contac~ with
the electrode, and the electrode an~ the c~l~us~tion are heated
bogether under pretsurc, for e ~ o ln a compresslon mould.
. 2~rticularly when the oonductive poly~er oompo d tion exhibits
PTC behavior, it ~ often des~rahl~ that Ln the final prcduct the
ccmposition sh~uld be crc~-linkod. Cr~3s-linking c~n be carried out
as a separate strp after thQ troat~ent b~ reduce oaQtact resistance; in
10- this c~se, cro3s-linking with aid of radla~ion ls preferr~d.
Alternatively crc~rl~nkinq c m be c~rriod out simultan~eously with the
said treat~ent, ~n which case che~ical cross-l~nking with the aid of
cross-linXing lnitiators such as peroxides is preferred.
me invention is illustrated by th~ foll~ ExamFles, scre
of which are canparative E~mple~.
In ea~ of the Example3 a strip heat ~QS prepared as
describff~ below The cor~uctive p~lymer ccrnFosition wa5 obtained by
blend$ng a ~edi~ den~ity poly~thyl~ne containing ~ antiox$dant wit~
carbon black ma~ter batch ~npr$s$nq an ethsylene,/ethyl aylate
c~lymer to giv~ ~ ~5it$0n containing the indicated perc&nt bv
weight of ca~on black ¦ ~Ihe m~t~ng po~nt o~ th~ ~os~t~on waa ~out f
~115-6~ me ccmp~$tian wa~ melt-extn~ed at a mclt temperatl~re of aba~t
180-C through a cro~ he~ d$e h~v$ng a circular or~ice 0 36 a~ in
diameter over a pair of ~randed ilvcr-coated oopper wires, each w$re
having a di m et r of 0.08 cm and o~mpr~inq 19 st:ands, and the a~es
of the wires being on a di3meter of the ori ice and 0.2 cm aFart.
Before reaching th~ crosJ-head die, th~ wires were pre-heated by
passing th~m through an oven 60 c~ long at 800-C. m e temperature of
the wires entering the die w~s 82-C in the comparative EXa~ples 1, 4
and 6, in which the speed of the wires thr3ugh the cven and the die
_g_
was 21m/m~, 165-C in Ex~e~ 2 ~ 7 and 193C in E~amples 3 ar~ 5.
me extr~ate wQ.~ then given an insl?lating j~cket by melt-
ex~ud~ng aro~d lt ~ layer 0.051 ~n thick of ~lorinat~ F~lyethylene
or an ethylene~te~ r~etl~lene copolymer. me coats~ ~xtru~at~ was
t~ irr~iated in order to os9-link the c~ ctive polyner
cc~sition.
EX~ES 1-3
5hese E~ople~, in which Exa:nE~e 1 is a c~nparative Ex2~21e,
domonstrate the inQ uence of ~inearity R~tio (LR) on Pcwer ~utput wh~n
the heater is subjected to te~perature changes~ In eadh Ex~mple, tbe
Linearity Ratio of the he~t ~as me~sured and the heater wQ~ then
oonnected to a 120 volt AC supply and the ambient temperature was
changed continuously over a 3 minute cycle, being raised frJR -37-C bo
65C over a pericd of 90 seconds and then reduced to -37C 3gain cver
the next 90 seconds.
m e peak power out2ut of the heater du- m~ each cycle ~as
measured initi~lly and at interval and expressed as a ~roportion ( ~ )
of the initial peak power output.
m e polymer ccm~vsition used in EXample 1 oontained a~out 25
carb~n black. T~he polymer compwitlon used in E~amçles 2 and 3
contained about 22~ carbon black.
12~651~7
~he results obt~ined are ~ in Table 1 below.
LE 1
. of C~cles ~Ec2mple 1 Example 2 Eurr~le 3
PN ~R PN ~R N
None 1 1.3 1 1.1
500 0.5 1.6 1.3 - 1 1
1100 0.3 2~1 1.2 - 1 1
ltOO - - 1.1 1.1 1 1
~a~arative E~nple
E~MP~ 4-7
These E~c~mple~, which are Sumnarised in Table 2 below,
da~ns~ate thc effect of pre-heating the ~ec~odes on t!le Linearity
Ratio arYI Pull Strength of the produ::t.
~.BLE 2
.
l; E:xample No. ~ Carbon_31ac!c Lirle ~ ~tio
~4 22 1.6
~2 1.0
*6 23 1.3S
7 23 1.1
20~Ca~arative E~t~nple
The r2tio of the pull ~trengt~s of the heater strlps of
B~eq 7 and 6 (P/PO) was 1.45.
