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 tape 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 tapes. One of the challenges is the dynamics changes of inorganic pigments from the thermal profile in manufacturing process and the uniformity of pigment dispersion in PTFE fine powder in order to reach maximum tinting strength.

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, Aerospace, Insulation, Wire, Cable

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. Comparing as other insulation material 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.

In addition, PTFE insulation also has many other merits, including:

  • Flame resistance and low smoke generation
  • Continuous service temperature range of - 260oC to 260oC
  • 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 special die. Calendar equipment is utilized to control the film thickness and lubricant is then removed through dry cans or solvent batch. 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 certain temperature for a period of time to fuse the PTFE layers. The temperature can be range from 240oC to 500oC 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 charged in PTFE through dry blending process. 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.

The purpose of coloring PTFE insulation is to trace and work on the wire later. Different colors mean differently in the field, for example, red and yellow color typically mean powder 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 only 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.

Table 1 Properties of PTFE and other Insulation Materials

Properties of PTFE and other Insulation Materials Chart

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 contains cadmium as the chemical components which is highly toxic. Cadmium red, yellow and orange have been familiar for many industrial applications by giving brilliant color, good permanence and tinting powder. More important for high performance applications, they can resist processing or service temperatures up to 3000oC. Cadmium pigments have excellent heat stability, light fastness, chemical resistance, non-migration and non-bleeding properties in polymers. However, excessive intake of cadmium can produce varieties of acute and chronic 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 Substance RoHS Limit Typical Use
Lead (Pb) 1000 ppm Solder, batteries, ceramic, components
Cadmium (Cd) 100 ppm II-VI compound, batteries, pigments
Mercury (Hg) 1000 ppm II-VI compounds, lighting, switches
Hexavalent Chromium (Hex-Cr) 1000 ppm Coatings for metal housings and chassis
Polybrominated Biophenyls (PBB) 1000 ppm Flame retardants for plastic
Polybrominated Diphenyl Ether (PBDE) 1000 ppm 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 the 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.

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 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 lot efforts from both 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. However, RoHS compliance in military and aerospace industries is a done deal. Once the focus starts moving from lead to cadmium, there would be no time left before every component suppliers are required to following the RoHS regulation.

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 Mulsell color limits 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 590oF for 72 h. No significant color change is allowed and the color has to meet the Mulsell color limits.

Table 3 Munsell Color Limits For UV Laser Markable Wire

Color

Hue

Value

Chroma

From

To

From

To

From

To

Black

2.5N

2.5N

7

8.5

N/A

N/A

Blue

5PB

7.5B

7

8

4

6

Green

2.5G

7.5G

7

9

2

6

Red

10RP

5R

7

8

4

6

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


CIELAB Color ModelAnother 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. CIE L* a b 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 each colored PTFE tape in L* a b values throughout the entire manufacturing process.


Figure 1 CIELAB Color Model

Development of Cadmium Free Red and Yellow Pigmented PTFE Tape

Among the inorganic pigments, cadmium pigments have bright, pure hues ranging from light yellow to orange, red and deep maroon-red. These pigments are manufactured through reaction of cadmium and sulphur atoms to give basic cadmium sulphide. The major component for the cadmium pigments are cadmium sulfides, zinc sulfides, and sulfoselenides. Cadmium sulphide with golden yellow color forms the basis for all cadmium pigments. When other metal sulphides or selenium is introduced, other desired shades can be attained. The use of zinc yields greenish yellow. Sulphoselenide oranges and reds form another series of pigments. With increasing selenium content, color changes from yellow to orange, red and finally dark red.

In order to meet the requirements in the processing condition for those colored PTFE tape, two thermal tests have to be conducted. First one is the sintering test where the PTFE tape is sintering at 1000oF for 1 mins, and the other is thermal stability test which is conducted at 590oF for 72 h to evaluate the thermal stability of the pigment in PTFE tape. In this study, many pigments involving different chemistry have been evaluated and tested. Cadmium free chemistry includes iron oxide, bismuth vanadate, nichel titanate, chrom titanate, cerium sulphide, etc. Some pigments were unstable in the sintering stage, showing streaking in the tape (see Figure 2A). This could be due to the pigment migration at the sintering temperature when PTFE starts melting at 327oC. After PTFE melts, the fibrillation created in the extrusion process will diminish and molten particles and fibrils begin to coalesce. Pigment particles starts penetrate into the PTFE molecules chains, followed by elimination of the voids. The dispersion of pigment particles in PTFE matrix is a complex phenomena in which blending efficiency, pigment particle size, surface chemistry, molecular interaction could all play important roles. Votalization is another phenomena in the sintering stage when the pigment particles are totally leached out the PTFE matrix because the extremely high mobility of inorganic molecular and high surface tension (see Figure 2C). After passing the first test, the second test it needs to pass is the thermal stability testing. As shown in Figure 2D-F, some pigments will fade slowly within 24 h and become lighter after 48 h. For thermal stable cadmium free chemistry, the pigments will disperse well in the PTFE tape and shows very good thermal stability with no bleach.

Figure 3 shows the color different between cadmium pigmented and cadmium free PTFE tapes after sintering.

High Temperature Sintering and Thermal Stability Tests for Pigmented PTFE Tapes
Figure 2 High Temperature Sintering and Thermal Stability Tests for Pigmented PTFE Tapes (A) streaking, (B) streaking/votalization, (C) votalization, (D) color fade after 1 h, (E) color fade after 24 h, (F) color fade after 48 h
Pigmented PTFE Tapes After high Temperature Sintering with Excellent Tinting Strength and Thermal Stability
Figure 3 Pigmented PTFE Tapes After high Temperature Sintering with Excellent Tinting Strength and Thermal Stability (A) cadmium red, (B) cadmium, yellow, (C) cadmium orange, (D) cadmium free red, (E) cadmium free yellow, (F) cadmium orange


To conduct the color matching based on Mil-Spec, CIELAB method was utilized to define each cadmium free colors. Figure 4 shows the L value for varieties of cadmium free inorganic pigments that we evaluated. For colored PTFE tape, the L values changes through the whole processing steps, from blending to extrusion, from wire wrapping to sintering. The L value is always hard to control in formulation because PTFE is a very good light reflector. It has been widely used in the optical detector, preventing loss of emitted light from scattering events. a value denotes the red/green value and b value the yellow/blue value, they can be used to specify the hue and chroma. Because the a and b value for a clear PTFE tape is close to zero, the color contribution in hue and chroma will come from the pigment directly. Pigment loading is another factor to consider when doing color matching. Figure 5 shows the comparison between cadmium red PTFE tape and cadmium free red PTFE tape, the L* a b values have been successfully used to evaluate the colored PTFE tape. Three dry pigments and three pigment dispersion have been evaluated for this study. Using the same method, we were able to evaluate the cadmium free yellow pigments and results are shown in Figure 6 and 7.

Value for Cadmium Free Red Pigmented PTFE Tape
Figure 4 L* Value for Cadmium Free Red Pigmented PTFE Tape
Values for Cadmium Free Red Pigmented PTFE Tape
Figure 5 a & b Values for Cadmium Free Red Pigmented PTFE Tape
Value for Cadmium Free Yellow Pigmented PTFE Tape
Figure 6 L* Value for Cadmium Free Yellow Pigmented PTFE Tape
Values for Cadmium Free Yellow Pigmented PTFE Tape
Figure 7 a & b Values for Cadmium Free Yellow Pigmented PTFE Tape

Development of Cadmium Free Orange Pigmented PTFE Tape

Cadmium yellow pigments (CdS) are manufactured by adding sodium sulfide to dissolve metallic cadmium or a cadmium salt solution to precipitate the fine particle raw colorant. Calcination at temperatures between 600oC and 700oC induces particle growth to 0.2 µm at which optimum tinting strength and hiding power are attained. In the manufacture of orange cadmium sulfoselenides, selenium is dissolved in the sodium sulfide solution in an amount consistent with the color required. Particularly brilliant hues are obtained, if the precipitated raw colorant undergoes brief heat treatment in molten salt at 800oC whereby an optimum particle size of 0.2 to 0.4 µm can be obtained.

Using CIELAB method, we could observe how the color values changes from yellow to orange to red as show in Figure. The orange color shows at the cross point at the two curve where a value is almost equal to b value. Many commercially available cadmium free orange pigments are evaluated, but the results are not satisfied. Instead of chemically incorporating the selenium to the pigment particle, we pre-blend the cadmium free red and yellow pigment together to create cadmium free orange pigment blend. Figure 8 shows the color values of cadmium contained orange pigments, by increasing the selenium content, the color changes from yellow to orange and then to red. The intersection between a and b value is the orange color where a value is equal or close to b value. Same trend was found in the cadmium free orange pigments blends.

CIELAB Color Values for Cadmium Pigments
Figure 8 CIELAB Color Values for Cadmium Pigments
CIELAB Color Values for PTFE Tapes Pigmented by Cadmium Free Orange Pigment Blends
Figure 9 CIELAB Color Values for PTFE Tapes Pigmented by Cadmium Free Orange Pigment Blends

Conclusions

Required by RoHS directive, cadmium pigments (red, yellow, orange) in the hookup electric wires have to be replaced by cadmium free pigments. As a result, the cadmium content in the PTFE insulation layer is not permitted to be more than 0.01% in weight. The newly developed cadmium free pigmented PTFE tapes have been evaluated through high temperature sintering and thermal stability tests. The cadmium free pigments and pigment blends showed no any streaking or votalization in the PTFE fine powder matrix throughout the entire processing steps. The color and tinting power of the sintered PTFE tapes were kept nearly equivalent before and after thermal aging at 590oF for 72 h. CIELAB method was successfully utilized to define the colored PTFE tapes. Other than chemically incorporating the selenium to the yellow pigment to form orange color, cadmium free orange pigments were developed by pre-blending red and yellow cadmium free pigments with very fine particles.

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