Summary
Target Industries: Semiconductor, Electronics, High-end Cabling
Problem
Regular skived PTFE Tapes only exhibit a tensile strength of 20-30Mpa. Tensilized tapes need to be 3-4X this value, with no loss of other properties except elongation.
Solution
Custom stretching and heating process to ‘lock in’ the molecular structure and enhance the tensile and electrical properties
Result
Tensilized PTFE tapes with 90-100Mpa tensile strengths
Introduction
PTFE skived tapes are unique among polymer films. Unlike regular thermoplastics, PTFE does not have any melt flow. This means that even at its melting point, rather than flow like a liquid, PTFE retains its shape, going into what is call a ‘gel state’. Considering this, there is no way to draw PTFE into a film as one would with a polymer like nylon or PVC. This necessitates the use of skiving, wherein a billet or pancake of PTFE is rotated against a sharp blade that effectively peels the top layer off the PTFE as a continuous film.
High precision skiving machines can produce films as low as 0.025mm in thickness. The advantage of these films is that the exhibit superior dielectric properties and high breakdown voltages. They also retain other properties of PTFE, such as temperature and chemical resistance, making them ideal in harsh, heavy-electrical environments.
Considering the structure of PTFE, the material is known for being soft and generally exhibiting high elongation. However, there are applications wherein such a high elongation is unnecessary, and it becomes possible to trade some of this property for higher tensile strength.
Property | Regular Skived PTFE Tape | High Tensilized PTFE Tape |
|---|
Tensile Strength | 20 – 35 MPa | 50 – 150 MPa |
Elongation at Break | 250 – 450% | 100 – 150% |
Dielectric Strength | 50 – 100 kV/mm | 80 – 150 kV/mm |
The Challenge
In order to improve the tensile strength, PTFE tapes need to undergo a molecular realignment. However, the manner in which this is done needs to be highly controlled. The main challenge is understanding how the material needs to be handled. Skived PTFE is made via a process of compression moulding, meaning the material, after skiving, is stronger in the longitudinal direction than the lateral. However, standard skived PTFE does not undergo any post-skiving processes to ‘work’ the material so that it can improve.
To understand how to do this, we decided that a combination of heat and mechanical load needed to be given. However, while PTFE is highly heat resistant, subjecting the films to temperatures above 250°C was causing the film to curl up and deform. Conversely, anything under 250°C seemed to be having no effect on the material.
Giving mechanical loads was also challenging. PTFE has a naturally low coefficient of friction, hence subjecting the tape to any clamping or frictional load was causing slippage.
This slippage also showed up later when testing the tensile properties. Due to the same, the tensile testing clamps were unable to hold on to the PTFE skived tape for the full duration of the test, meaning the tape would slip free before the tensile strength could be fully calculated.
The Solution
A combination of fixtures and equipment were developed and/or modified to achieve the following:
- A special arrangement that allowed the tape to maintain its shape under high temperatures along with a special spooling arrangement to ensure it remain in a high-tension state even as it cooled
- A customised clamping arrangement to allow for the mechanical loads to be applied in a way that was uniform and could be easily customised
- A special clamping arrangement to hold the tape during tensile testing.
Developing the above was only the starting point. Beyond this, multiple combinations of load and temperature were tested until we arrived at a result.
It should also be noted that for the sake of maximising the tensile properties, we initially used modified PTFE. Although more expensive, the uniformity of the material gave us confidence to try multiple parameters. Gradually, as we began achieving the results, we applied the process on regular grades and saw that the same properties were being achieved. However, in order to truly maximise the tensile strength, we had to do one more step.
- Create a specialised sintering cycle to improve the billet characteristics.
Regular sinter cycles offer properties that generic PTFE would be expected to have. Here, we were attempting to increase the crystallinity of the material, which needed a special cycle that would optimise for the same.
The Outcome
Our initial tests were underwhelming, offering strengths only in the range of 40-45Mpa. However, as the process improved, we started seeing values in the range of 60-70Mpa. With the final fine tuning and the introduction of the special sinter cycle, we finally broke 95Mpa, which is what we were targeting. It is possible that we can push this further to reach 150Mpa. More work may be needed to achieve the same.
Conclusion
For the longest time, we assumed that PTFE properties were static and that only a higher quality of raw material could enhance them. This experience showed us that process improvements can induce not just incremental changes but alter the base properties of a polymer entirely.
Read More
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Case Study: PEEK Gears - Evolution in High-precision Machining Capabilities2.
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