Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Polymer Radomes (Radar Enclosures)

The evolution of radar technology over the past few decades has allowed it to be applied in multiple applications, even as the cost of the technology has reduced. In order to support the smooth functioning of radar antennae, radomes (quite literally, Radar-Domes) were developed to ensure that the antennae are kept free from interference – either environmental or wireless. The importance of the radome cannot be stressed enough. In areas such as defence, it is imperative that the antennae are flawless. Missiles, for example, are guided using wireless communication and any interference that alters the missile’s trajectory could be catastrophic.

Still, considering that radar and wireless technologies can, by definition, pass through solids, there is a lot of design freedom on the radome’s composition. However, certain materials do perform better and are therefore preferred.

Radome design

The cover of a sensor is critical of its construction and can have an important influence on sensitivity, radiated antenna pattern, and immunity to vibrations. Radome design involves minimizing microwave reflection at the surface of the cover. A poorly constructed radome can even cause unwanted sensitivity on the backside of the sensor. The cover material can also act as a lens and focus or disperse the radar waves. This is why it should have a constant thickness within the area used for transmission.

A radome should be designed in order to minimize its influence on sensor sensitivity as well as on the field pattern of the radar antenna. Any reflection caused by the radome leads to a degradation of the sensor characteristics. For FMCW radars, proper radome design is even more important than for simple Doppler Radars: reflexions near the antenna cause strong feed through of the FM signal to the IF output.

An imperfect radome reflects parts of the transmitted waves. As a radome can never be perfect, relative movements (vibrations) between antenna and radome will lead to large signal levels at the radar transceiver. These signals mostly look like normal Doppler signals caused by moving targets and can lead to malfunction of the sensor system.

Materials used to construct radomes must be dry and electrically isolating. Any metallic parts – including coatings that may contain metals – must be avoided. Primarily, the following can be adhered to:

  • Radome cover must not be metallic

  • No plastic coating with colours containing metallic or carbon particles

  • The distance between cover and the front of Radar sensor >= 6.2mm

  • The optimal cover thickness is 3-4mm

  • Vibrations of the radar antenna relative to the cover should be avoided, because this generates signals that can trigger the output


Preferred materials for this application include the following:

Material

Permittivity (εr)

 Dissipation factor (tanδ)

Polycarbonate

 2.9

 0.012

ABS

 2.0-3.5

 0.0050-0.019

PEEK

 3.2

 0.0048

PTFE (Teflon®)

 2

 <0.0002

Plexiglass®

 2.6

 0.009

Glass

 5.75

 0.003

Ceramics

 9.8

 0.0005

PE

 2.3

 0.0003

 

PTFE Radomes

PTFE (Teflon®) is among the most effective materials available for radome construction. The material is fully machinable – implying that the uniform wall thickness required can be maintained throughout the part. In addition to this, PTFE is very robust in protecting against environmental forces over the long term. PTFE exhibits such low values for both permittivity and dissipation that it has virtually no effect on the antennae’s functioning.

Improvements in CNC machining – including CAM technology – have allowed increasingly complex radomes to be developed. These are usually made using high-purity virgin PTFE (Teflon®), which ensures that the values mentioned in the table above are attained without any hassle. Any impurities, discolorations or non-uniformity in the material would render it unsuitable for this application. Hence, care needs to be given that only the highest grade of resins are used in manufacturing the same.

Anti-Static PTFE Tapes

Among the myriad properties of PTFE that make it such a sought-after material, electrical resistance rates as one of the more popular. Because of its extraordinary dielectric strength and high breakdown voltage, PTFE is an invaluable addition in heavy electrical applications. As such, the following products are used across industries:

1. Skived PTFE tapes for heavy insulation 

2. Expanded PTFE (ePTFE) Tapes for cable wrapping

3. PTFE Radomes

4. PTFE transducer covers

5. PTFE insulation blocks

6. PTFE battery separators

These are but a few products that are commonly used. We often find that applications where electrical discharge is likely to be high benefit from a component made from PTFE to ensure that the equipment remains safe and does not leak current, causing harm.

Anti-static PTFE

The downside to the extreme insulative properties of PTFE lie in the build-up of static charge. Most applications do not find the build-up of static electricity to affect their process. However, certain assemblies – especially those where the equipment is being used in environments where flammability is high – require the charge to dissipate through the insulation so that sparks or static bursts do not occur. In such a situation, pure PTFE can cause problems, as it is such a strong insulator, that it does not allow the static charge to run through it.

To mitigate this problem, anti-static PTFE materials can be made, which employ conductive materials such as carbon to give mild conductive properties to the PTFE and allow it to discharge static build-ups through the carbon mixed into it.

Anti-static PTFE Tapes

Increasingly, applications that require PTFE tapes have started using anti-static tapes in areas where static build up can be an issue. However, most applications are very specific about the resistivity of the tape. Too much resistivity and you risk static build up; too little and you end up with a material that is too conductive to insulate effectively.

The base property of PTFE gives a surface resistivity of 1014 ?. For most conductive applications, this value needs to be reduced to 104 ?. In order to achieve this, the base filler of carbon needs to be adjusted. A lot of this final property depends on both the base property of the virgin PTFE (different grades will vary from the base value mentioned above) and the conductive properties of the carbon additive itself. Various types of carbon will offer different levels of reduction in surface resistivity, implying the percentage of filler needs to be adjusted accordingly.

Another complication that arises due to the addition of carbon is that there can be a physical discharge of materials from the surface of the PTFE. Because PTFE and carbon are merely combined as a mixture, there is the likelihood that fine particles can come loose from the surface of the PTFE. For many high-purity applications, this can be a showstopper. Hence the quantity of the conductive filler needs to be minimized, while also allowing the conductive properties to be met. One material that has emerged as effective in this regard is Vulcan. While standard grades of conductive carbon need to be mixed to the extent of 10-15% into the PTFE (85% PTFE, 15% Carbon), with Vulcan, the same level of conductivity can be achieved with as little as 1-2%. However, as Vulcan is expensive, its incorporation is restricted to applications where there is a stringent need for no particle discharge to happen.

Electrical properties of pure virgin PTFE 

General properties

Density

ISO 1183

2,16

g/cm³

Transparency

 

Opaque

 

Mechanical properties

Stress at yield

ISO 527

10

MPa

Tensile strength

ISO 527

20-25

MPa

Elongation at break

ISO 527

350

%

Tensile modulus (Flexural Modulus)

ISO 527

420

MPa

Flexural strength @ 3.5% deflection

ISO 178

14

MPa

Ball pressure hardness

ISO 2039-1

28

MPa

The standard for ball pressure hardness

 

H358 / 30

 

Hardness Shore (A/D) or Rockwell (R/L/M)

ISO 868, ISO 2039-2

D55

-

Izod notched impact strength 23 °C

ISO 180/4A

185

J/m

Friction against steel without lubrication

 

<0.1

-

Abrasion relative to the pressure

 

420

(µm/km)/MPa

Electrical properties

Dielectric constant 50 Hz

IEC 60250

2,1

-

Dielectric constant 1 MHz

IEC 60250

2,1

-

Dissipation factor 50 Hz

IEC 60250

0,5

10-Apr

Dissipation factor 1 MHz

IEC 60250

0,7

10-Apr

Dielectric strength

IEC 60243-1

7.5-24

kV/mm

Thickness for electric strength

 

3

mm

Volume Resistivity

IEC 60093

1.00E+14

? · m

Surface resistivity

IEC 60093

1.00E+14

?

Creep Resistance (Comparative Tracking Index)

IEC 60112

600

-

Thermal properties

Thermal conductance

ISO 22007

0,24

W/K m

Specific heat

IEC 1006

0,96

J/g K

Linear thermal expansion along/cross to direction of flow

ISO 11359

130-200

10-6/K

Melting point

ISO 11357

327

°C

Heat distortion temperature A

ISO 75 HDT/A (1,8 MPa)

50

°C

Heat distortion temperature B

ISO 75 HDT/B (0,45 MPa)

121

°C

Short time use temperature

 

300

°C

Continuous use temperature

 

260

°C

Minimal use temperature

 

-200

°C

Other properties

Humidity absorption at 23°C/50%

ISO 62

<0,1

%

Water absorption

ISO 62

<0,1

%

Flammability UL 94

IEC 60695-11-10

V-0

-

Limiting oxygen index

ISO 4589

95

%



 

Tensile Testing of PTFE

The tensile strength is among the most important properties of any polymer. It offers an insight into the ability of the polymer material to withstand loads – primarily pulling loads – and along with tensile elongation, tells us to what extent the polymer will yield before breaking. While other properties such as specific gravity, Young’s Modulus and Compressive Strength also feature as important, most manufacturers start with checking tensile strength and then move on from there.

For this reason, it is vital that a full understanding of the methods to check tensile strength are understood before conducting the same.

How is a tensile test done?

In its most basic form, the tensile test involves pulling a material apart until it breaks. You then look at the load that the material was able to sustain before breaking, as well as how far it elongated before it broke.

Different materials have different standards on how to ensure that this test is conducted uniformly and using the parameters defined.

1. In any tensile test, there are a few critical parameters to consider:

2. The speed of the test – namely the speed at which the material is pulled apart

3. The thickness of the test specimen - The shape of the specimen – usually a dumbbell shape, but the exact shape may vary across materials

Tensile test for PTFE

For PTFE, the ASTM standards form the basis for most of the testing done. Specifically, standards pertaining to PTFE include the following:

1. ASTM D4894 – used for virgin (unfilled) grades of PTFE

2. ASTM D4745 – used for PTFE compounds such as glass filled, bronze filled, and carbon filled PTFE

Both the above standards make reference to ASTM D698 with regards to tensile testing.

Because both standards refer to ASTM D698 for tensile testing, most test laboratories will test as per this standard, without verifying with the parent standard for specifics pertaining to PTFE.

As a material, PTFE tends to behave differently to other polymers. As such, both the ASTM D4894 and ASTM D4745 make specific references to the tensile test and outline the differences between the regular testing as per D698 (which makes no mention of PTFE) and the testing needed for PTFE.

The key differences can be found on all three parameters listed above. Namely:

1. Thickness

While ASTM D698 suggests thicknesses of 3mm and above, the PTFE specific standards recommend the specimen should be no more that 1mm thick (+/-0.25mm)

2. Speed of testing

ASTM D698 recommends standard testing speeds of 5mm/minute, whereas PTFE specific standards give speeds of 50mm/minute. One of the properties of PTFE – as we have experienced during the manufacture of expanded PTFE – is that it behaves differently when stretched rapidly as against being stretched slowly. As a result, it is conceivable that the standards would make allowance for a higher stretch rate when considering PTFE as a material.

3. Shape of the specimen

In our own experience, the shape of the specimen does not change the tensile testing results significantly. Nonetheless, the dumbbell shapes defined by ASTM D698 differ slightly from the shape recommended by PTFE specific standards.

It should also be remembered that equipment used for testing should be able to clamp the PTFE specimen adequately to ensure there is no slippage during testing. Given that PTFE has a low coefficient of friction, the risk of slippage can be high, and this can distort the final readings. At the same time, too aggressive a clamp can cause the material to fracture at the point of clamping itself. In such a case, the test would need to be repeated as the reading would be spurious.

Further, PTFE as a material can experience elongation in excess of 500%. In such a situation, the equipment would need to be capable of this much travel, as the ideal reading would be taken at the point when the material breaks. However, in the event that the required tensile and elongation properties are met, even the reading on a specimen that does not break can be considered acceptable.

Tensile properties of PTFE

In general, PTFE will exhibit tensile strengths ranging from a low of 10Mpa all the way up to 35-40Mpa. Much of this depends on the type of filler used, the moulding and sintering parameters as well as the purity and quality of the base resin. Elongation values will also range from a little as 150% to as much as 550%. Here again, the composition and quality of the material plays a key role. Although varying the parameters does not – in our opinion – greatly alter the final readings one obtains on tensile and elongation properties, it is nonetheless best practice to follow the same as per the standard. It is possible that using a different speed setting, thickness and specimen shape causes an early breakage and rejects the material that would otherwise have been accepted under settings prescribed by the standards.