Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

The Effect of Low Temperatures on PTFE Component Dimensions

One of the most challenging elements of machining PTFE components for export markets is factoring the effects of temperature on the material when it moves into colder climates.

Since we do all our machining in Bangalore, India, where the room temperature varies between 22-32 Degrees Celsius (on average), we need to be constantly mindful of the dimensional shrinkage that would happen when machined parts are shipped to colder countries.

In many cases, this problem is not a huge one – since the tolerance on the part may be high enough that even after shrinkage it would still fall within the acceptable band for that dimension. A tolerance of +/-0.25mm, for example, could be machined with a plus side bias of 0.15-0.2mm – which is easily maintained on a CNC machine. After shrinkage, even if the dimension reduces by 0.2-0.3mm (not unheard of as we will later demonstrate), the part would still be acceptable when inspected at the client’s works prior to assembly.

The real challenge lies in accommodating much closer tolerances. In our own experience, we encourage customers to design the part keeping in mind a tolerance of +/-0.05mm at most. Often, as the customer may have dealt mainly with metal parts, they expect that PTFE would also conform to the same dimensional yardsticks as metal (which can be machined to tolerances as fine as 1 micron). In reality – PTFE is a much softer material, which undergoes the following changes during machining, affecting it’s ability to be attain very close tolerances:

  • Stress build up in material due to tool hardness
  • Deformation of material due to heat from machining
  • Burrs forming on part which may need to be manually removed, affecting tolerance

The closest tolerance we have managed to maintain on PTFE has been +/-0.012mm – which was done on a component made from PTFE+15% glass fiber, having an outer diameter of 19.04mm.  We did this by experimenting with different combinations of tools, RPM, feed rates and programs, until the dimension was consistently within the tolerance range. However, when the part was shipped to the client in Canada, the trial lots failed due to being undersize. Eventually, through trial and error, it was found that a dimension of 19.08-19.10 was needed in India, in order for the part to be within tolerance in Canada.

While this worked out well eventually, in many cases, customers are not willing to experiment with trial lots – especially if their requirement is urgent. This has led us to seriously consider the practical implications of shrinkage and how we can make an educated guess on dimensions so as to avoid rejection/ rework and/or minimize trials.

The study

Virgin PTFE theoretically experiences a 1.3% variation in dimension between 0 and 100 Degrees Celsius. Plotting this as a linear progression around the dimension of 19.04 would give us a chart like this:

In other words, given the room temperature in Canada being ~10-15 degrees cooler than in India, a shrinkage of 0.03mm could be expected when the parts reached Canada. We expected that for glass filled PTFE, this may not be quite as high – as glass itself may not be as susceptible to dimensional deviation based on temperature.

In order to check this, we machined identical components from different grades of PTFE, having the outer diameter of 19.07mm at room temperature and cooled them down to well below 0 degrees Celsius. We measured this dimension when the pieces were taken out of the sub-zero environment and then allowed them to sit at room temperature, measuring the outer diameters and temperatures at regular intervals to plot a curve of dimension versus temperature.

The grades we used were the following:

For each grade – 5 identical parts were machined and their dimensions were measured along the same point. The dimension considered was 19.07 +/- 0.02mm. The parts were first measured at room temperature (about 25 degrees Celsius) and then put into a sub-zero environment for 4-5 hours. Each part was then taken out individually and measured again at fixed intervals of 10 minutes. The aim was two –fold: (1) to observe the extent of shrinkage due to the cold and assess the rate of expansion as the part warmed up at room temperature (2) To gauge whether the part, when left at room temperature overnight, regained it’s original dimension.

The charts below show the results for each grade:

It was observed that the virgin material experienced the highest shrinkage (0.9% over a temperature range of 40 degrees Celsius). Both Glass and Bronze filled PTFE experienced lower shrinkage (0.5% over a temperature range of 30-32 degrees Celsius). It is also interesting to note that virgin PTFE reached a much lower minimum temperature. While the filled grades were recorded as having temperatures of  -4 to -5.3 degrees at their lowest, the virgin material was recorded with a minimum of -11 degrees – despite being subjected to the same sub-zero environment prior to measurement.

 

Additionally, all three grades reverted to within 0.01mm of their original dimensions when left overnight to warm under room temperature. This suggests that the dimensional change is linked purely to the ambient temperature and that there is no observable stress build-up in the material due to the cold which causes it’s original dimension to alter permanently (at least, not in the range of -15 to +30 degrees Celsius).

All three materials lend themselves to a high R-squared straight-line graph. While we accept that this is a fairly simplistic relation to assume, the R-squared changes only marginally when we try to introduce more complex equations. Furthermore, while the relation between the dimension and the temperature may not be a strictly linear one – for the purpose of practicality, we believe that it serves quite well. In other words, assuming a 0.2% shrinkage for every 10 degrees in temperature (for virgin PTFE) would imply that a 20mm dimension would need to accommodate a plus side tolerance of 0.4mm. This is in line with our own trial and error conclusions thus far, when exporting to colder countries.

Finally – it could be pointed out that the same experiment could be carried out at higher temperatures to gauge whether PTFE continues to expand the same way in the other direction. However, as the bulk of our export destinations are in fact colder countries, we have not looked at this right now. Perhaps the same could be taken up in a separate post.

Mapping the PTFE Price Increase

While much has been said about the causes and implications of the PTFE price escalation, we felt it necessary to go through our archives and chart out the exact extent to which the prices have changed.

The chart below shows the price per Kg in US$ for three standard grades – Virgin PTFE, Glass Filled PTFE (15%) and Bronze Filled PTFE (40%). In addition, we have included a table showing the total and monthly growth in prices.

Needless to day, the growth has been unprecedented. In Virgin PTFE, a nearly 8% increase in prices every month has put the industry in a state where there is no breathing time between processors getting new pricing information and passing on that information to the customers.

Most processors are well aware of the effect this has had on their businesses. The main issue has been convincing customers regarding the price increase and furthermore making them aware that the trend may be expected to continue. In addition to this, there is the impact on repeat business, as clients withhold contracts which would have otherwise spanned their requirements over a full year – since processors are unable to commit to prices for more than a one month horizon.

PTFE pricing revisited ? inevitabilities in long term supply and demand

When we started this blog, our aim was two-fold:

1. To inform and educate readers about PTFE, it’s applications and derived products
2. To serve as a platform for clients and other end-users to understand PTFE better and make informed decisions regarding their applications

However, it seems to have been the articles on pricing which have brought most of our traffic as regardless of how important the applications of PTFE are, it is – understandably – on pricing that most questions currently centre.

We therefore want to look at pricing again, just to see if any new information gleaned over the past couple of months helps us understand the situation any better than we did earlier.

Since our last blog on the impact of Fluorspar on PTFE prices, we have – like all other processors – been praying for stability. We haven’t been praying for a reduction in prices – that would be optimism to the point of pure irrationality. However, if prices could simply stabilize – even for a few months, it would give us some time to re-group, re-assess and possibly resume normal operations.

However, the pricing fluctuations have been coupled with lesser-known events in the background and together, these effects are causing the stabilization process to take much longer than earlier assumed.

We would like to take a look at some of the news floating in the market at present. We can vouch that since these are from rather reliable sources, we are inclined to believe them and therefore base our outlook on their implications:

1) Fluorspar shortage is no longer an issue

A supplier who regularly sources semi-finished PTFE from a Chinese manufacturer told us this anecdote: The supplier approached the manufacturer with the offer to supply R22. The proposed arrangement was that the PTFE manufacturer could then supply the resin manufacturer with R22 (assumed to be in very short supply) and in return procure resin at a discounted price. The supplier was shocked to hear that the resin manufacturer declined – saying that they had ample R22 to meet their production needs. This does lead us to believe that although the Fluorspar story may have started the PTFE price frenzy, it is now not playing as significant a part.

2) European resin manufacturers have re-allocated resources away from PTFE

This was partly confirmed by a representative from DuPont, who stated that their company was slowly coming out of PTFE resin manufacture, as long-term competition against Chinese suppliers was not feasible for them. The resulting effect, we hear, was that most of the European resin manufacturers have sub-contracted their PTFE business to Chinese resin suppliers. Since the realization for PTFE resins in Europe is much higher – the European price has become the new acting price across the global market.

This impact does throw some light on why the PTFE prices have increased so drastically all over the world. On the one hand, we have a supply constraint, as European manufacturers no longer compete in the market. At the same time, you have a huge supply-demand mismatch as European demand for resins stays the same and this drives up prices.

3) Russian suppliers are in a state of flux

From what we have heard, Russia has two main companies who manufacture PTFE resins, one of which acquired the other. The combined company is said to be undergoing some transition issues and management is also contemplating moving away from PTFE and into ETFE. The result has again been to constrain supply, impacting prices in the process.

4) Pricing set to stabilize within the next 2-3 months

Obviously, many are hoping that things will settle down sooner than this, but considering the extent of changes occurring across the market, one might expect that it would take no less than a few months to stabilise.

Our own local the supplier – who increased prices by another 30% in the month of July 2011, assured us that this would be the last-but-one, if not the last, price revision from their side. The current price we are getting is US$26.5 per Kilo for virgin PTFE resin. From what information we have from our European counterparts, it appears that local rates there are around the same price – so it does look like some sort of balance has been reached.

For those thinking about the long term implications of all this, we can infer the following from what data we have already collected:

  1. High prices are here to stay. If there is one thing that all this has shown us, it is that the demand has stayed strong enough despite the price escalation. This has justified the price hike for resin manufacturers from a business standpoint
  2. Long term, quality will improve. Although it looks like Chinese companies will be doing most of the manufacturing of PTFE resins, if they are supplying through companies like DuPont and 3M, the quality controls will most probably be more stringent.
  3. Volumes in PTFE will shrink. Although we have not seen a significant amount of substitution away from PTFE, there are murmurs of new materials and possible replacement materials in some areas. For the most part, we continue to believe that as a material, the extensive spectrum of properties offered by PTFE makes it a difficult material to shift out from. However, we do expect that at least 15-20% of the volumes in PTFE would slowly shift to other polymers such as PA66 and UHMWPE. Nonetheless, we can take comfort in the fact that a 15-20% fall in volumes when combined with a 100-200% increase in prices still implies an overall growth in the industry in value terms.
  4. Repro is here to stay. Not that anyone though that reprocessed PTFE would go away, but we do believe that the acceptance of recycled material in many applications (due to the price implications) would bring about some regularization in the market, with manufactures offering transparency on the extent to which reprocessed PTFE is used and possibly on the properties it could be expected to exhibit. Again – this would be a good thing from a quality standpoint, as buyers of semi-finished PTFE would at least know exactly what they were getting.

To conclude – we have, like most other processors, been trying to make the best out of a situation that has been completely out of our hands. We have faced a rather torrid 15-18 months, so if the end were 2-3 months away, we would look forward to that. In the mean time we would recommend planning one day at a time, because there is no telling what might happen during the next week or month.