Among the myriad properties of PTFE that make it such a sought-after material, electrical resistance is 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 10^14. For most conductive applications, this value needs to be reduced to 10^4. 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	%

 

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 PTFE tensile testing 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 the tensile properties of 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 on tensile testing.

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.

The advancements in polymer technology have resulted in myriad different plastics, each offering its own unique property. While some polymers are preferred for their strength, others may have specific resistance to certain chemicals, making it the material of choice in an industry where such chemicals are prevalent. A case in point of this is PPS (Polyphenylsulfone). PPS is a relatively brittle polymer, which usually requires a glass fibre reinforcement in order to maintain its structural stability. However, PPS is especially resistant to the chemicals used in pulp and paper industries, requiring that components used in the equipment for pulp and paper be made from this polymer.

While many properties have been tested and improved continually, it has always been the durability of the polymer that has remained a mainstay of its preference. Traditionally, OEMs and equipment manufacturers have always prided themselves on making products that can stand the test of time. Be it a car or a pair of shoes, quality and durability have always played the most prominent role in defining the industry leaders. However, this is suddenly changing.

Shortening product life cycles have started placing questions around the importance of durability. What use is a car that can stay fuel efficient and low maintenance over 10 years, if the consumer is going to buy a new car within 3 years anyway? Nowhere is this phenomenon more pronounced than in the electronics industry. Mobile phones, smart watches and even laptops are no longer needed for more than 18-24 months, before the consumer upgrades to a new variant.

The shortening of the life cycle has an impact on the input materials used. Devices no longer need high-performance materials, which are expensive and drive up the final cost of the equipment. Furthermore, with the ever-changing designs, a component needs to be made with as low an upfront cost as possible, since any one-time tooling and development charges cannot be amortized easily over volumes over time.

In the polymer space, this has created both a dilemma and an opportunity. Polymer components are typically of two variants:

1. Injection moulded parts – where the part is made by injecting molten polymer into a die that has the final shape of the component needed
2. Machined parts – where the part is turned or milled from a rod or sheet

Injection moulded parts are traditionally cheaper, when you only consider the marginal cost of making the part. Not only does the injection moulding use only as much polymer as needed, it also creates complex shapes within seconds and a single die may be capable of housing multiple cavities, so the process is high in productivity.

Machining is expensive, as you are effectively removing excess material from a rod or sheet. Furthermore, it is a length process that only gets longer as the complexity of the part increases.

Machining, however, wins against injection moulding on two accounts. For one, the cost of making the die can be both expensive and time consuming. Making a good die can often take over a month and cost upwards of US$1500 for a single die. In addition to this, the tolerances achievable on injection moulding are not always close. Once the die is made, if the part tolerances are not as per the drawing, either the OEM needs to accept the new dimensions as they are, or the component manufacturer must re-make the die, which is again expensive and time consuming. Machining, on the other hand, can easily attain tolerances of within 20 microns, if done properly. Furthermore, any change in drawing or revision only requires a simple modification of the programme on the CNC machine – which is free of cost and can be done in minutes.

So, even though machining yields a more expensive per-part cost, device manufacturers are slowly accepting that machined components are a better option for short product life cycle parts. Considering the mobile phone market operates on barely an 8-12-month product life, there is little time to invest in making a mould, testing the parts and moving to production. It is simpler for the OEM to utilise the expertise of a good machine shop, where both the polymer and the final component can be changed as required.

While machining is certainly taking off as a preferred option for making polymer parts, the grade of polymer has also been reviewed and revised accordingly. Polymers like PEEK, PCTFE and Polyimides are highly durable, but come at a steep price. For comparison, the price of PEEK would be nearly 20-40 times the price of cheaper polymers, such as Nylons and Polyacetal (Delrin). Considering that high-performance polymers offer, amongst other things, durability as a key property, there is a shift away from the more expensive plastics to those that offer lesser durability at a fraction of the price.

In our own experience, we have seen that while one part is running for an existing generation of equipment, another set of parts is already under development for the next generation. This pipeline allows the OEM the time needed to test and modify the parts as required, so that the time to market for the next generation of devices is shortened as much as possible. The ability to adapt and cater to this changing dynamic is based on a company’s capability to understand and work with many different polymers. Being able to take on a new polymer – based on what the client specifies – and immediately develop techniques to achieve the desired close tolerances on the same is a skill that is slowly acquired. By working with nearly 30 different polymers, Poly Fluoro Ltd. has developed a wealth of understanding on how each family of plastics behaves. This puts us in an ideal position to capitalise on the changing approach of today’s industries.