Development of RoHS Compliance Colored PTFE Tape for Aerospace
Electric Wire Insulation
Jeffrey Zhang and Charles Williams
Technetics Group, an EnPro Industries Company
10633 W Little York, Bldg 3, Suite 300, Houston, TX 77041
(713) 983-4210
jeffrey.zhang@plastomertech.com
In addition, PTFE insulation also has many other merits, including:
Abstract
As one of the leading dielectric materials used for insulation in
aerospace electric wire and cable, polytetrafluoroethylene (PTFE)
has excellent dielectric constant, low dissipation factors, wide
service temperature range, and high frequency stabilities. Applied in
hookup wire for electronic equipment, PTFE tape is wrapped to the
conductor in the unsintered state, and the whole construction is then
sintered, giving a uniform insulation with good electrical properties.
PTFE insulation layer can be colored by pigments in dry form or as
dispersion in the blending stage, but only inorganic pigments can be
used because almost all organic pigments will completely degrade
at the high processing temperature. Among the varieties of
pigments, cadmium pigments (red, yellow, and orange) have been
widely used in colored PTFE tapes for aerospace electric wire
insulation, providing brilliant colors with good permanence and
tinting power. Today, almost all the aerospace wire manufacturers
have claimed to be fully compliance with Restriction of Hazardous
Substances Directive (RoHS) since the July 2006 European
deadline, exemptions still exist in the EU documentation including
cadmium colored PTFE insulation. One of the challenges is the
stability of inorganic pigments through the thermal profile in
manufacturing process and the uniformity of pigment dispersion in
PTFE fine powder in order to reach maximum tinting strength.
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Flame resistance and low smoke generation
Continuous service temperature range of – 260 °C to 260 °C
Resistance to all common chemicals and solvents
Moisture resistance and volume resistance
Ability to color by inorganic pigments
Laser-markability by titanium dioxide
The unsintered PTFE has been widely used for electric hookup wire
in aerospace and military industries. It is produced by cold extrusion
process, in which fine powder PTFE is first blended with a lubricant
and then extruded through a die. Calendar equipment is utilized to
control the film thickness and lubricant is then removed through a
drying process. [1] To produce hookup wire in electronic equipment
for the aerospace and military industries, the unsintered PTFE tape
is spirally wrapped around the conductor with a minimum overlap
of 50% of the tape width to form a shield construction. The whole
construction is then sintered at elevated temperature, for a period of
time, to fuse the PTFE layers. The temperature can range from 240
°
C to 500 °C depending on the insulation thickness, the gauge of the
metal conductor, the speed of the production line and the length of
the sintering oven. Typically, pigments are mixed with PTFE
through dry blending process. A defect free dispersion is very
critical for the wire wrapping process because any defects with
undispersed spots could lead to edge lifting, folding, tape breaking,
or even electrical breakdown of the insulation. Meanwhile, the
pigment loading has to be as low as possible to minimize the
potential arc tracking.
In this study, methodology developed to evaluate the thermal
stability of cadmium free pigments in PTFE tape and CIELAB color
modeling to predict color using pigments blends will be discussed.
Key Words: PTFE, RoHS Compliance, Color, Cadmium Free,
The purpose of coloring PTFE insulation is to trace and identify
later for the field service. Different colors mean differently, for
example, red and yellow color typically mean power cable, and
orange means testing wire. The rule is “the brighter the color, the
higher the voltage”. Many general aviation aircraft are wired with
white wire in order to save money. However, most of the major
avionics manufacturers have established their unique color codes
for their aircrafts and offer them in the installation manuals.
Aerospace, Insulation, Wire, Cable
1. PTFE in High Performance Wire and
Cable
PTFE is one of the leading dielectric materials for use as insulation
in high performance wire and cable. Compared as other insulation
materials like polyvinyl chloride (PVC), polyethylene (PE), nylon,
rubber, PTFE has excellent dielectric constants, low dissipation
factors, wide service temperature range, and high frequency
stabilities. Table 1 lists relevant electrical properties for some
common insulation materials.
Table 1 Properties of PTFE and other Insulation Materials
PTFE
FEP
PI
PVC
PE
XLPE
EPDM
Silicone
Dielectric Constant
1.3-2.1
2.1
3.5
3.5-12
2.3
2.3
3.0-3.5
3.0-3.5
Dissipation Factor (1KHz)
<0.0001
<0.0001
0.003
0.07
0.0002
0.0004
0.004
<0.01
Volume Resistivity (Ω.cm)
1019
1018
1017
1013
1015
1015
1016
1011
2. Cadmium Pigments and RoHS
Compliance
Cadmium pigments, including red, yellow and orange, have been
widely used in aerospace electric wires for a few decades. They are
a class of pigments that contain cadmium as the chemical
components which is highly toxic. Cadmium red, yellow and orange
have been used for many industrial applications providing brilliant
color, good permanence and tinting strength. More importantly for
high performance applications, they can resist processing or service
temperatures up to 3000 °C. Cadmium pigments have excellent heat
stability, light fastness, chemical resistance, non-migration and nonbleeding properties in polymers. However, excessive intake of
cadmium can produce a variety of acute and chronic health effects,
leading to kidney damage, disturbances of calcium metabolism, etc.
RoHS (Restriction of Hazardous Substances Directive) was adopted
in February 2003 by the European Union. The RoHS directive took
effect until July 1, 2006. This directive restricts the use of six
hazardous materials (see Table 2) in various types of electronic and
electrical equipment. For cadmium, the maximum permitted
concentrations are 0.01% or 100 ppm by weight of homogeneous
material. This means that the limits do not apply to the weight of the
finished product, but the homogeneous component. For example,
the insulation layer can be separated mechanically from an electric
wire, therefore, the concentration limit will apply to the insulation
layer not the electric wire. For a composite electric wire, the bottom
layer is formed by polyimide and the surface layer by PTFE. Thus,
this limit will apply to the outmost PTFE layer of the wire.
Table 2 Substances banned by RoHS Directive
Banned Susbtance
RoHS Limit
Lead (Pb)
1000 ppm
Cadmium (Cd)
100 ppm
Mercury (Hg)
1000 ppm
Hexavalent Chromium
(Hex-Cr)
1000 ppm
Polybrominated
Biophenyls (PBB)
Polybrominated
Diphenyl Ether (PBDE)
1000 ppm
1000 ppm
Typical Use
Solder, batteries,
ceramic,
components
II-VI compound,
batteries, pigments
II-VI compounds,
lighting, switches
Coatings for metal
housings and
chassis
Flame retardants
for plastic
Flame retardants
for plastic
In the high performance wire and cable industries, the pigment
concentration in PTFE tape is much higher than 0.01%. According
to RoHS directive, everything that can be identified as a
homogeneous material must meet this limit. Therefore, if it turns out
that the PTFE tape contains 1% cadmium pigment, in which 80% of
the composition is cadmium, then the entire wire would fail the
requirements of the directive.
3. RoHS Compliance in Aerospace and
Military Industries
In the US and elsewhere around the world, original equipment
manufacturer (OEM) within the military and aerospace industries
has been almost universally negative in responding to RoHS
compliance. Most of the products manufactured within these
industries are either excluded from the RoHS legislation, or are
covered by approved exemptions. Although the majority of the
focus on RoHS has been put on lead free electronics for the
industry, there are still a lot efforts and collaborations between
material suppliers and wire manufacturers on developing cadmium
free electric wires compliance with RoHS.
So far, most of the high performance wire manufacturers are
claiming RoHS compliance on their products but exemptions still
exist for the red, yellow, orange pigmented PTFE in both melt
extruded and wire wrapped articles. One major reason is that the
testing results from all the cadmium free pigmented PTFE don’t
meet the crucial requirements for this application. The color
stability at processing temperature and the color itself didn’t
perform as well as the cadmium pigmented PTFE tape. There is no
doubt that RoHS compliance will be completely accepted in
military and aerospace industries. However, there would be no time
left, once the focus starts moving from lead to cadmium, before
every component suppliers are required to follow the RoHS
regulation.
4. Testing Methods Development
To manufacture colored PTFE tape, pigments are added in the dry
blending step with PTFE fine powder. The nature of white color for
PTFE powder makes the color matching and formulation extremely
unpredictable. The PTFE powder has very high crystallity of 98%.
When the crystallinity change during the sintering process, the
background color changes and make the pigment color more
brilliant. Therefore, it is pretty complex to develop a testing method
to conduct color matching in order to meet the Munsell color limits
(see Table 3) specified in the Mil-spec. From dry pigments to
powder blend color, from tape color to wire color after sintering, the
colors are quite different in each stage. After these, the wire needs
to pass a thermal stability test at 310 °C for 72 h. No significant
color change is allowed and the color has to meet the Munsell color
limits.
Table 3 Munsell Color Limits for UV Laser Markable Wire
Color
Hue
Value
From
To
Black
2.5N
Blue
5PB
Green
2.5G
Chroma
From
To
From
To
2.5N
7
8.5
N/A
N/A
7.5B
7
8
4
6
7.5G
7
9
2
6
6
Red
10RP
5R
7
8
4
Yellow
5Y
10Y
8
9
4
6
Brown
2.5YR
7.5R
7
9
2
4
Orange
10R
2.5YR
6
7
8
10
Violet
2.5P
7.5R
7
8
4
8
Gray
2.5N
2.5N
7
8.5
N/A
N/A
Another challenge for this study is to measure the colors. Each color
has its own distinct appearance, based on three elements: hue,
chroma and value (lightness). Munsell system used in the Mil-Spec
assigns numerical values to the three properties. CIELAB method
(see Figure 1) is a common system measured by a
spectrophotometer. L* defines lightness, a means the red/green
value and b the yellow/blue value. Using this color methodology,
we are able to express
w
e
each colo
ored PTFE tape in L* a b valuues
thhroughout the enntire manufacturiing process.
Figure 1
(A)
(B)
(C)
(D)
(E)
(F)
CIEL
LAB Color Mod
del
5. Develop
5
pment of Ca
admium Frree Red
a Yellow
and
w Pigmente
ed PTFE Ta
ape
Among the inorganic pigments, cadmium pigm
A
ments have brighht,
p
pure
hues ranginng from light yellow
y
to orangge, red and deep
m
maroon-red.
These pigments are manufactured thhrough reaction of
c
cadmium
and sullphur to give baasic cadmium suulphide. The majjor
c
components
for the
t cadmium pig
gments are cadm
mium sulfides, zinnc
s
sulfides,
and sullfoselenides. Cad
dmium sulphidee develops golden
y
yellow
color whhich forms the basis for all caadmium pigmennts.
W
When
other mettal sulphides orr selenium are introduced, othher
d
desired
shades can
c be attained. The use of zinnc yields greeniish
y
yellow.
Sulphoseelenide oranges and reds form another series of
p
pigments.
With increasing selen
nium content, coolor changes froom
y
yellow
to orange,, red and finally dark
d red. [2-4]
Inn order to meet the
t processing reequirements for colored
c
PTFE tappe,
tw
wo thermal tests have to be cond
ducted. First one is
i the sintering teest
w
where
the PTFE tape is sintering at 538 °C for 1 min, and the othher
iss a thermal stabiility test which is
i conducted at 310
3 °C for 72 h to
e
evaluate
the therm
mal stability of the pigment in PTFE
P
tape. In thhis
s
study,
many piggments involving
g different chem
mistries have been
e
evaluated
and tessted. Cadmium free
fr chemistries include
i
iron oxidde,
b
bismuth
vanadatee, nickel titanate,, chrome titanatee, cerium sulphidde,
e Some pigmeents were unstab
etc.
ble in the sinterinng stage, showinng
s
streaking
in the tape (see Figurre 2A). This couuld be due to thhe
p
pigment
migratioon at the sinterin
ng temperature when
w
PTFE starrts
m
melting
at 327 °C.
C After PTFE melts,
m
the fibrillaation created in thhe
e
extrusion
process will diminish and molten paarticles and fibrrils
b
begin
to coalescee. Meanwhile, piigment particles penetrate into thhe
P
PTFE
molecular chains, followed
d by eliminationn of the voids. Thhe
d
dispersion
of piggment particles in
i the PTFE maatrix is a complex
p
phenomena
in which
w
blending efficiency, pigm
ment particle sizze,
s
surface
chemistrry, and molecular interactionn could all play
im
mportant roles. Volatization
V
is an
nother phenomennon in the sinterinng
s
stage
when the pigment
p
particless are totally leacched out the PTF
FE
m
matrix
because thhe extremely hig
gh mobility of innorganic molecullar
a high surface tension (see Figu
and
ure 2C). After paassing the first test,
thhe second test itt needs to pass is
i the thermal sttability testing. As
A
s
shown
in Figure 2D-F, some pigm
ments will fade slowly
s
within 244 h
a become lighhter after 48 h. For
and
F thermally staable cadmium frree
c
chemistry,
the pigments disperse well in the PTF
FE tape and show
ws
v
very
good therm
mal stability with
h no leaching. Fiigure 3 shows thhe
c
color
difference between cadmiu
um pigmented and
a cadmium frree
P
PTFE
tapes after sintering.
Figure 2 High Temperature
T
S
Sintering
and Thermal
T
Stability Tests for Pigmented
P
PTF
FE Tapes (A) streaking,
s
c
fade
(B) sttreaking/volatization, (C) volatization, (D) color
after 1 h, (E) color fa
ade after 24 h, (F) color fade
e after 48 h
(A)
(D)
(B)
(E)
(C)
(F)
Figure 3 Pig
gmented PTFE
E Tapes After high
h
Temperature Sinte
ering with Excellent Tinting Strength
and
d Thermal Stab
bility (A) cadmium red, (B) ca
admium
yello
ow, (C) cadmiu
um orange, (D)) cadmium free
e red, (E)
c
cadmium
free yellow, (F) cad
dmium free orange
To conduct the color matching based on Mil-Spec,
T
M
CIELA
AB
m
method
was utilized to define eaach cadmium freee colors. Figuree 4
s
shows
the L* value
v
for varieeties of cadmiuum free inorgannic
p
pigments
that wee evaluated. For colored PTFE tape,
t
the L* valuue
c
changes
throughout the processing steps of blending, extrusioon,
w wrapping, annd sintering. Thee L* value is always hard to contrrol
wire
inn formulation beecause PTFE is a very good light reflector. It has
h
b
been
widely usedd in the optical deetectors, preventing loss of emitted
liight from scatterring events. The hue and chromaa can be correlated
w the a value denoting the red
with
d/green color spaace and b value thhe
y
yellow/blue
colorr space respectiv
vely. Because thhe a and b valuues
f a clear PTFE tape is close to zero, the color contribution
for
c
in huue
a chroma will come
and
c
from the piigment directly. Pigment
P
loadingg is
a
another
factor to consider for co
olor matching. Figure 5 shows thhe
relationship betw
ween a and b valu
ues for the cadmiium free red PTF
FE
p
and thrree pigment disppersions have been
taape. Three dry pigments
e
evaluated
for this study. It was proved that CIE
ELAB method has
h
b
been
successfullyy used to evaluatee the colored PTF
FE tape. Using thhe
s
same
method, wee were able to evaluate
e
the caddmium free yelloow
p
pigments
and resuults are shown in
n Figure 6 and 7.
Figurre 6
L* Value for Cadmium Free Yellow Pigmented
P
e
PTFE Tape
Fig
gure 7
Figure 4
L** Value for Cad
dmium Free Re
ed Pigmented
PTFE
E Tape
a & b Values for Ca
admium Free Yellow
Y
Pig
gmented PTFE
E Tape
6. Developmen
nt of Cadm
mium Free Orange
O
mented PTF
FE Tape
Pigm
Cadmiuum yellow pigm
ments (CdS) arre manufacturedd by adding
sodium
m sulfide to disssolve metallic caadmium or a caadmium salt
solutionn to precipitate the
t fine particle raw colorant. Caalcination at
temperaatures between 600
6 and 700°C innduces particle growth
g
to 0.2
µm whhere optimum tinnting strength andd hiding power are attained.
In the manufacture
m
of orange
o
cadmium
m sulfoselenides, selenium is
dissolveed in the sodium
m sulfide solutioon in an amounnt consistent
with thee color required. Particularly brilliant hues are obbtained when
the preecipitated raw colorant
c
undergooes brief heat treatment
t
at
800°C where
w
an optimuum particle size of
o 0.2 to 0.4 µm is
i obtained.
Figure 5
a & b Values
s for Cadmium
m Free Red
Pigmented
d PTFE Tape
c
values
Using CIELAB methood, we could observe how the color
change from yellow too orange and theen to red by inccreasing the
m content. Figuure 8 shows thee color values of
o cadmium
selenium
pigmennts, from left to right,
r
the color changes
c
from yeellow to red.
The oraange color shows at the intersecttion of the two curves where
the a value
v
is almost equal to the b value.
v
For this study,
s
many
commeercially availablle cadmium freee orange pigm
ments were
evaluatted, but the resullts were unsatisffied. An alternattive method,
instead of chemically incorporating thhe selenium to the
t pigment
particlee, is to pre-mix the cadmium frree red and yellow pigment
toogether to createe cadmium free orange pigment blends. As show
wn
inn Figure 9, colorr of pigment blen
nds can be contrrolled by adjustinng
thhe loading levell of one pigmen
nt. The intersectiion between the a
a b values gavve the orange collor where the a value
and
v
was equal or
c
close
to the b vaalue. This trend for cadmium freee pigment blennds
w exactly samee as that observeed in cadmium pigments
was
p
in which
thhe selenium molecular is incorpo
orated chemicallyy.
pigmennts were developped by pre-mixinng red and yelloow cadmium
free piggments with veryy fine particles.
R
8. References
[1] S. Ebnesajjad, Fluoroplastics, Voluume 1 - Non-Mellt
Prrocessible Fluorooplastics, Plastic Design Library,, Norwich,
NY
Y 2000.
[2] G.. Buxbaum andd G. Pfaff, Induustrial Inorganic Pigments,
W
Wiley-VCH
2005..
[3] P. Lewis, Pigmentt Handbook, Vol.1, Wiley, New York
Y
1988.
VCH 2009.
[4] E. Faulkner, High Performance Piggments, Wiley-V
9. Piictures of authors
a
Figure 8 CIE
F
ELAB Color Va
alues for Cadm
mium Pigments
s;
Left to Right: color cha
anges from yelllow to red
Figure 9 CIELAB Colo
or Values for PTFE
P
Tapes
Pigmente
ed by Cadmium
m Free Pigmen
nt Blends;
Left to Right: color cha
anges from yelllow to red
Jeffrey Zhang is a R&D
D engineer for Teechnetics Group, an EnPro
Industriies Company. Hee has extensive experience
e
in PTF
FE
processses, application and
a product devellopment, and is currently
c
leadingg the developmennt and commerciaalization of new products for
high peerformance wire and
a cable applicaations. He holds B.S. in
Chemiccal Engineering from
f
East China University of Sccience and
Technoology and a Ph.D
D. in Chemical Enngineering from Clemson
C
Universsity.
7 Conclus
7.
sions
Required by RooHS directive, cadmium
R
c
pigmeents (red, yellow
w,
o
orange)
in high performance
p
elecctric wires need to be replaced by
b
c
cadmium
free piigments. As a reesult, the cadmiuum content in thhe
P
PTFE
insulation layer is not perrmitted to be moore than 0.01% in
w
weight.
The newlly developed cad
dmium free pigm
mented PTFE tappes
h
have
been evaluuated through color
c
matching, high temperatuure
s
sintering
and theermal stability teests. The cadmiium free pigmennts
a pigment blennds showed no any
and
a streaking or volatization in thhe
P
PTFE
fine powdder matrix throug
ghout the entiree processing stepps.
T color and tinnting strength of the sintered PTF
The
FE tapes were keept
n
nearly
equivalentt before and afterr thermal aging at
a 310 °C for 72 h.
C
CIELAB
methodd was successfu
ully utilized to define
d
the colorred
P
PTFE
tapes. Rathher than chemically incorporatinng the selenium to
thhe yellow pigm
ment to form oraange color, cadm
mium free orange
Charless Williams is a Srr. Technology Manager
M
for GGB
B Bearings
Technoology, an EnPro Industries
I
Compaany, and currentlly manages
fluoroppolymer composiite development for
f the next geneeration of
self lubbricated materialss and processing methods. He haas over 20
years exxperience in Polyymer Processingg and Specialty Chemical
C
Manufaacturing technoloogy. He holds a B.S.
B in Chemicaal
Engineeering from Renssselaer Polytechnnic Institute.