Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

Mapping the PTFE Price Increase - An Update

We have been receiving many mails asking us to map or at least project the PTFE price trend going forward. Since our last post on PTFE pricing was about a year ago (Feb 2012), it is a valid question to ask whether there has been any further volatility in this market and what that implies for prices as a whole.

To lay any suspense to rest right away – we can firmly say that prices have indeed been stable this past year and it is due to this stability that our own interest in analyzing the prices has dimmed somewhat. However, that is not to say that things would continue along this vein indefinitely. PTFE is a complex material and the dynamics involving its manufacture and sale are constantly in flux – meaning that the next shock may just be around the corner at any time. So we would like to look at some of the buzz surrounding the industry in a hope to at least demystify the future to some extent.

Prices are expected to remain stable throughout 2013

This is the general consensus as of now and is due to two primary factors:

  1. There is a general slowdown in global demand (much in line with most other industries) that makes it risky for resin manufactures to experiment with pricing like was done early in the price escalation of 2010-11
  2. There was a significant over supply of resins when prices were high and this led to huge inventories which manufacturers are still offloading

In some areas it is believed that there is still some scope for prices to fall further. However, it is most widely accepted that the current rates are stable and should be for the foreseeable future

China still a key player – but not the only price maker

Back in 2010, it was largely a price war between manufacturers that led to very low PTFE prices. Much of this was driven by China – where the abundance of fluorspar and the support from their government allowed Chinese PTFE manufacturers to scale up very quickly.

Although China remains a key player still, a few factors are affecting their economics and scale right now:

  1. The government support has reduced for PTFE resin manufacturers in China. With the clamping down of R22 within China (for environmental reasons), there is a subsidy of US$0.5 per Kg of R22, which is no longer being offered by the Chinese government.  Owing to this, Chinese manufacturers have had to pass on this cost increase of about US$2 per Kg on to PTFE processors
  2. Due to quality issues with Chinese resins, many OEMs have started specifying that their parts be made from resins such as DuPont, Daikin or AGC. Many semi-finished PTFE processors have also shifted away from Chinese resins due to the instability of the material. It was long believed that China reserved the good quality resin for their domestic manufacture and preferred to dump the off spec grades into other countries (one of the reasons why the anti-dumping duty on Chinese resins was upheld by the Indian government). This has hit the volumes of Chinese resin manufacturers
  3. Another key issue is repro material. While India earlier had significant imports of Chinese semi-finished PTFE materials, most of this was repro material but was being passed off as 100% pure virgin PTFE in the local market. This high intake of semi-finished PTFE was affecting the local semi-finished manufacturers as well as local resin suppliers. However – owing to the major quality issues with repro and the lack of accountability and transparency in the percentage of repro being incorporated, many companies have had to stop procuring semi-finished PTFE from China and have started buying domestically – where it is easier to monitor quality and also return material if found defective. This has led to a resurgence in domestic PTFEsemi-finished goods production and also an increased off-take from local resin manufacturers.

With China on the back foot due to the reasons listed above, it may be safe to say that price manipulation and/or competition is for the time being not a threat – since it is usually with China that most of these issue do arise. Hence, the current view is that of stability – and we should enjoy that while it lasts.

Delrin - the machinist's best friend

In a world of specialized plastics requiring immense tensile strength or high wear resistance or minimal coefficient of friction, Delrin holds its own against the more versatile polymers such as PEEK and PTFE.

Our own experience with Delrin began with the PTFE price increases in 2010-2011, as we scrambled to find substitutes for PTFE to offer clients, without compromising too much on properties. As we have already stated in earlier articles – finding a true substitute for PTFE was futile. However, despite our attempts to push UHMWPE and PA66 as replacements (materials we were more familiar with), it was ultimately Delrin which clients were most comfortable in adopting.

What is Delrin?

Delrin (brand name of DuPont) is also commonly referred to as POM (Polyoxymethylene), polyacetal, or simply acetal. The names all refer to a polymer that is characterized by a high tensile strength, high stiffness, low coefficient of friction and excellent dimensional stability. In addition to its properties – Delrin is a relatively inexpensive material compared to PTFE and even PA66. This makes it a sought after choice in machined component development – as the parts are dimensionally very stable and significant trials can be done without being too expensive.

Properties of Delrin

  • Delrin is characterized by its high strength, hardness and rigidity to ~40 °C
  • In its natural form, it is a white (opaque) plastic, although it is easily pigmented and often available in a variety of colors
  • Delrin has a specific gravity of 1.410-1.420 g/cm3
  • As a homopolymer it is 75-85% crystalline with a melting point of 175°C, while as a copolymer has a slightly lower melting point of 165–175°C
  • It has a relatively low coefficient of friction of 0.2 – much higher than PTFE, but still suitable for a wide number of engineering applications
  • Delrin is resistant to a wide variety of chemicals including alcohols, aldehydes, esters, ethers, hydrocarbons, agricultural chemicals, and many weak acids and bases. This ability is even more impressive when we consider that even under harsh chemical environments, Delrin does not lose its dimensional stability
  • Electrically, Delrin rates slightly below PTFE, but is nonetheless a very useful substitute. Its dielectric constant (~3.5) is only slightly higher than PTFE (~2)

Advantages of Delrin:

  • High mechanical strength and rigidity
  • Toughness and high resistance to repeated impacts
  • Long-term fatigue endurance
  • Excellent resistance to moisture, gasoline, solvents, and many other neutral chemicals
  • Excellent dimensional stability
  • Good resilience and resistance to creep
  • Natural lubricity
  • Wide end-use temperature range
  • Good electrical insulating characteristics

Due to its versatility, Delrin finds uses in a number of applications including:

  • Automotives
  • Industrial equipments
  • Consumer goods
  • Medical equipments
  • Electrical equipments

Machining Delrin

As a machined item, Delrin is particularly easy to work with. With PTFE, we need to consider the softness of the material and also its sensitivity to temperatures, with nylons and UHMWPE, we need to be careful of the part melting during machining, with PEEK, the tool itself can break, if we do not control the RPM. However, Delrin is surprisingly accommodating as the part retains its stiffness, but is still soft enough that the tool is able to work through the plastic. In addition, the dimensional stability post machining is also excellent. While we have had instances of PTFE parts being under tolerance when shipped to colder climates, the same is not an issue with Delrin parts.

Applications and Considerations for PTFE Seals

As a sealing element, PTFE has proven itself many times over. PTFE is used in seals because it encompasses all the properties essential for a good sealing element, mainly:

  • High wear resistance
  • Low coefficient of friction
  • Moderate hardness (allowing for better overall mating with metal parts)
  • Durability – both with temperature as well as corrosion

While these properties are not new to us, every material has a limit to how much it can withstand. Furthermore, every grade of PTFE offers something different to the sealing application. Understanding these limits and differences gives us a better understanding into choosing and applying PTFE seals to best suit the requirement.

Sealing is vital to almost any mechanical assembly. It serves to both retain fluid within the assembly and allows the assembly to function freely. A good sealing material – such as PTFE, needs to be elastic enough to close gaps and assist with the fluid retention while strong enough to take the wear load applied on it by (usually) metal mating parts. Still, within any assembly, there is likely to be some trade off between fluid retention and durability, and this is where the choice of grade becomes important. Typically, the following metrics needs to be studied:

Surface Finish

PTFE wears off in layers, and will usually deposit a coating on the mating surface. In general, it is easy to attain a surface finish of as high as Ra < 0.4 on a virgin PTFE part. However, once we introduce other materials such as glass, carbon, graphite or bronze into the mix, there is a huge drop in the finish. We have successfully attained a finish of Ra < 1.2 on PTFE+15% Glass seals – but going below this is always a challenge.

For the mating part, the surface finish is somewhat more important – as it is usually a metal and can wear the PTFE out significantly faster if not properly finished. When the metal surface is rough, more wear occurs until the crevices and valleys within the metal are filled with PTFE. PTFE will wear in direct proportion to the surface finish. Testing shows that the life of the seal is doubled when the finish is improved from 16rms to 8rms.

Surface finish also affects the sealing ability of PTFE. A rough finish creates microscopic “line of sight” channels allowing a flow path through mating parts. Hence, when sealing gases with small molecules, such as, hydrogen, helium, or oxygen, a 2-4 rms is highly recommended.

Hardness

When the mating part is hardened (via heat treatment or plating), there is a significant improvement in the life of the seal. Typically, when a hard and soft surface are in contact, there is an exchange of ions, which can lead to adhesion. This reduces the effectiveness of the seal. Improving the surface hardness of the metal part can control the adhesion.

PV Value

PV is an often-quoted metric for all PTFE grades. It offers a trade-off between the pressure that the PTFE can take, against the speed at which the mating part is sliding against the PTFE.  Understanding PV is key to understating whether the PTFE grade being considered at would be able to withstand the combination of load and RPM involved.

Disregarding PV values would almost certainly lead to a failure in the seal to perform. We have received many requests to look into the replacement of standard phosphor-bronze bushings, bearings and seals with PTFE grades. In most cases, PTFE looks to be a perfect substitute along most metrics. However, when we look at the pressure it can withstand under high RPMs, PTFE is not always suitable.

Types of seals

Given the diversity in automotive and mechanical applications, a number of different PTFE seal dimensions have been developed – each with it’s own unique property. When we cross these dimensions with the different PTFE grades, we end up with potentially hundreds of sealing options. Thus, choosing the right seal is important and a lot of thought needs to go into the same, before a decision is made.

The spring-energised seal is a sealing device consisting of a PTFE ‘energized’ by a corrosion resistant metal spring.  Put simply – as PTFE is a soft material, it can be easily deformed by the metal parts surrounding it. The spring acts as a strengthening medium – allowing the PTFE to take loads while also applying force on the sealing surfaces to create a tighter fit and ensure no leakages. The spring also provides resiliency to compensate for seal wear, gland misalignment or eccentricity.

Types of such spring energised seals include:

Finger Spring:

This is mainly used in dynamic applications, has good sealing and a low coefficient of friction. It is recommended for surface speeds up to 250ft/min.

Coil Spring:

This is designed for more static or slow dynamic applications. It is not as flexible as the finger design – owing to the fact that the spring is coiled and more rigid as a result. However, it is significantly better than the finger design in sealing – due to the uniform pressure applied on all sides by the coil spring.

Double Coil Spring:

A more augmented version of the single coil – this is designed for purely static applications, such as cryogenics. The increased load applied by the double coil significantly improves sealing ability.

O-ring Energised:

This can be used in both static and dynamic applications and offers a good balance between the seal-ability of the coil energised seal and the flexibility of the finger spring. It is typically incorporated in areas where metallic springs cannot be used due to compatibility issues.

Rotary Lip Seals

Lip seals are used primarily to seal rotary elements such as shafts and bores. They provide a self-lubricating medium between (usually) two metal elements – allowing for both smooth rotation and good sealing. Common examples include strut seals, hydraulic pump seals, axle seals, power steering seals, and valve stem seals.

Lip seals may be designed with or without springs – depending on the application.

The examples shown above are merely indicative of the basic designs available in PTFE seals. In truth, each of the above types of seals may be expanded into many variants, depending on the exact requirements of the mating elements involved in the OEM designs. Furthermore, each may be provided in any of a number of grades of PTFE compounds available.

Choosing a PTFE compound for your PTFE seals

The grade of PTFE is a critical choice in the design of the seal. We have touched elsewhere on the variants and properties offered by the commonly used fillers in PTFE. In a nutshell – glass offers stiffness and creep resistance; bronze and molybdenum di sulphide offer wear resistance, but increase the coefficient of friction; carbon and graphite offer wear resistance and dimensional stability.

In our experience, a mixture of glass and molybdenum di sulphide offers the ideal sealing properties for most applications. However – the exact grade is usually a choice made by the OEM, based on what information we are able to provide.