Unravelling Polymers

The Definitive Blog on Polymers by Poly Fluoro Ltd.

A Comparison of High-Performance Polymers

With new developments being constantly introduced in the polymer space, exciting new products are always entering the market. However, as the scale remains low, most new plastics remain prohibitively expensive but for the niche applications for which they may have been created. Through all this, the erstwhile stalwarts - PTFE and PEEK - have retained much of their effectiveness as increased scale and breath of application has allowed them to become more cost-effective and compete with existing medium-performance polymers on high-volume parts.

Apart from PTFE and PEEK, it is also important to look at FEP and PFA. Both these polymer variants were an offshoot of PTFE. Indeed, few realise that the trade name “Teflon”, which is used so interchangeably with PTFE does in fact cover PFA and FEP as well.

The reason for developing PFA and FEP was quite simple: PTFE has a very low melt flow and hence cannot be injection moulded. This limitation makes PTFE a material that can only be machined, which in turn makes complex parts and high-volume parts a difficult prospect when using PTFE. FEP and PFA both have lower melting points and have melt flows which allow for injection moulding. However, it is important to note that in this trade-off, both polymers surrender various properties, making PTFE the superior material in terms of absolute performance and versatility.

The below table offers some key comparisons between these four polymers, in order to offer an understanding of each one’s advantages, disadvantages, and applications.

 

Name of the Polymer   POLYETHER -ETHERKETONE POLYTETRAFLUOROETHYLENE POLYTETRAFLUOROETHYLENE GF 25 % PERFLUOROALKOXY ETHYLENE PERFLUOROALKOXY ETHYLENE GF 20 % FLUORINATED ETHYLENE - PROPYLENE FLUORINATED ETHYLENE - PROPYLENE 20 % GF
Trade name/ Typical name   PEEK PTFE PTFE 25 % GF PFA PFA 20 % GF FEP FEP20 % GF
Type of the Polymer   Thermoplastic Thermoplastic-Thermoset Thermoplastic Thermoplastic Thermoplastic Thermoplastic Thermoplastic
Advantages   PEEK is a high performance thermoplastic with the characteristics common to this group - strong, stiff, hard, high temperature resistance, good chemical resistance , inherently low flammability and smoke emission. It is pale amber in colour and usually semi-crystalline and opaque, except thin films are usually amorphous and transparent. It also has very good resistance to wear, dynamic fatigue and radiation Outstanding chemical resistance. Low coefficient of friction. High continuous use temp. 180 Cº . Very high Oxygen index. Higher modulus and surface hardness than PTFE. Improved creep resistance, dimensional stability and wear compared with PTFE. Melt processable, has similar chemical resistance to PTFE combined with the highest temperature resistance of melt processable fluoro plastics. Self-extinguishing. Retains room temperature stiffness and strength at elevated temperatures better than FEP. Excellent toughness. Significant increase in HDT and moderate increase in tensile strength compared with unmodified grades of PFA. Very high impact strength. Excellent high frequency electrical properties. Melt processable. Good weathering resistance. Significantly increased tensile strength, HDT and flexural modulus compared with unmodified grades of FEP. The mechanical properties of moulded components can be anisotropic.
Disadvantages   It is difficult to process and very expensive. Low strength and stiffness. Cannot be melt processed. Poor radiation resistance. Lower impact strength, lower tensile strength and more expensive than unmodified PTFE. Stiffness and strength similar to those of PTFE at room temperature. More expensive than PTFE. Decreased elongation at break and notched izod impact strength compared with unmodified grades of PFA. Very expensive, with the lowest strength and stiffness of all the fluoro plastics. Low HDT at c 50°C ( 120°F ) accompanied by poor wear resistance. Elongation at break and notched izod impact strength are reduced compared with unmodified FEP. The mechanical properties of moulded components can be anisotropic.
Applications   Applications include flexible printed circuit boards (film), fibres and monofilaments, injection moulded engineering components and items used in aerospace and radiation environments. Filled grades, including ones designed for bearing-type applications, are also used. Bearings, Chemical vessels linings, pipe and valve linings, gaskets, diapharms, piston rings, high temp. electrical insulation. As a coating of non stick applications. Wear pads, piston rings, and microwave oven rotating platforms. Heater cables, chemically resistant linings for pumps and pipes etc. that require a higher temperature resistance. Chemical plants. Coatings, protective linings, chemical apparatus, wire coverings, glazing film for solar panels. Valves, electrical components and equipment for chemical plants.
PROPERTIES UNIT              
Density g/cm³ 1.26 - 1.32 2.15 2.25 1.6 2.2400000000000002 2.1 2.2000000000000002
Surface Hardness RR M 99 [Rockwell] SD 63 SD72 SD60 SD 68 RR45 RR65
Tensile Strength Mpa 70-100 25 17 29 33 14 40
Flexural Modulus Gpa   0.7 1 0.7 0.7 0.6 5.5
Notched Izod Imapact strength kJ/m 0.85 0.16 0.12 A.06+ 0.7 1.06+ 0.2
Linear Expansion /Cº x 10?5   15 12 21 13.5 5 5
Elongation at Break % 50 400 250 300 4 150 2.5
Strain at Yield %   70 N/Y 85 N/Y 6 N/A
Max. Operating Temp. 250 180 180 170 170 150 150
Water Absorption % 0.1 - 0.3 0.01 0.01 0.03 0.04 0.01 0.01
Oxygen Index % 35 95 95 95 95 95 95
Flammability UL94 V 0 @ 1.5 mm V0 V0 V0 V0 V0 V0
Volume Resistivity log ohm.cm 10¹5-10¹7 18 15 18 18 18 14
Dielectric Strength MV/m 19 @ 50 ?m 45 40 45 40 50 13
Dissipation Factor 1kHz 2.9999999999999997E-4 1E-4 3.0000000000000001E-3 2.0000000000000001E-4 1E-3 2.0000000000000001E-4 5.0000000000000001E-4
Dielectric Constant 1kHz 3.2-3.3 @ 50Hz-10Khz 2.1 2.8 2.1 2.9 2.1 2.5
HDT @ 0.45 Mpa › 260 121 125 74 160 70 260
HDT @ 1.80 Mpa 160 54 110 30 150 50 158
Material Drying hours @ Cº 4-6 HOURS @ 200° NA NA NI NA NA NA
Melting Temp. Range 360-420 NA NA 360-420 360-420 340-360 350-380
Mould Shrinkage % 0.8 - 1.5 NA NA 4 0.8 2.5 0.4
Mould Temp. Range 175 - 200 NA NA 50-250 50-250 50-200 50-200

Polymer Bearings - Self-lubrication Solutions for Critical Applications

About Polymer Bearings

In an already crowded bearing space, polymer bearings have made their mark for a host of different reasons. As a result, an area that was once dominated by steel and phosphor bronze is increasingly giving way to polymers such as PTFE, PEEK, POM, and Nylons, where the sheer breadth of grades and fillers allows for a whole range of properties tailored to match the end-application and offer a solution that far exceeds what metal bearings were able to hitherto provide.

The advantages and disadvantages of polymer bearings against metals can be shown on the chart below:

Polymer Bearing

Advantages

Disadvantages

Lightweight

Limited load capability

Fully customisable

Can be expensive

Self-lubricating

Limiter temperature range

Easy to replace

 

 

As shown above, metallic bearings are typically preferred where the loads and possibly the temperatures are much higher. Here too, however, certain polymers such as PEEK and Polyimide (Kapton), can bear enormous loads and remain functional in temperatures of 300°C+. However, such polymers come at a price and are therefore limited in applications such as aerospace and medical, where cost may not be a key criterion.

However, for many applications, polymer bearings find that their advantages are highly sought after. Key among this is the ability to self-lubricate. Polymers such as PTFE, POM, and UHMWPE - to name just a few – offer dry-running capabilities which greatly reduce the need for external lubrication. This is especially valuable in consumer goods, where the structure of the device or appliance is such that the user will not have access to the moving parts. Similarly, in certain industrial applications, self-lubrication ensures minimal down time and greatly reduces the wear and load due to the build-up of friction.

Types of Polymer Bearings

Polymer bearings come in various shapes and sizes and can be either machined from a drawing or reverse-engineered from an existing part. Some of the typical bearings offered by Poly Fluoro Ltd. include:

1. Flange bearings
Flange bearings are designed to handle both axial and radial loads. In some designs the flange is also used as a locating mechanism to hold the sleeve in place.

Flange bearings can be machined either from stock rods or moulded. Polymer grades used would include PTFE (usually with a glass or bronze filling), PEEK (virgin or carbon filled), PPS (usually with a glass filling), and POM.

Flange bearings require a little more machining to the housing but can solve the unique load conditions of a shaft and some type of thrust surface.
 

2. Mounted bearings
Mounted bearings are machined with a double flange in order to sit within a pillow block. These bearings can be fabricated using several different plastic bearing materials to improve wear and reduce or eliminate lubrication.
 

3. Thrust bearings
Put simply, thrust bearings are washers made from any number of materials such as PTFE, PEEK, PPS, POM, Nylons, or Polyimides. They are generally thin, easy to install and prevent metal on metal contact in any thrust load conditions. They are easy to use and do not require lubrication of any kind in most conditions.

Although the design is simple, there is a need to machine the part so that the surfaces are perfectly parallel. This is where Poly Fluoro excels.
 

4. Sleeve bearings
These are the most common bearings, with a simple ID, OD, and length. However, as with the washers, care needs to be taken to ensure the tolerances are tight. Where most manufacturers would only offer a 100 Micron tolerance on linear dimensions, Poly Fluoro is able to go down to as low as 10 Microns in some cases.

The bearings are designed to carry linear, oscillating, or rotating shafts. The key to successfully designing a plastic sleeve bearing is paying attention to temperature, P, V and PV ratings for the material and match it with your application.
 

5. Spherical bearings
Spherical bearings are designed to allow for shaft misalignment, as they can rotate in two directions. Spherical bearings typically support a rotating shaft in the bore that calls for both rotational and angular movement.

Using self-lubricating polymers with very low static coefficients of friction, Poly Fluoro is able to ensure that even minor variations in alignment are immediately accommodated by the bearing to allow for non-stop performance.

While the above bearings are most common, application engineers are constantly finding new areas in which to apply the bearing properties of polymers. Ultimately, any application with repeated motion will benefit from a polymer bearing as it offers an unmatched ability to reduce wear and friction over a very long period of running time.


Related Posts

1. Solar Tracker Bearings - Considerations for Design and Manufacture

2. Cantilever Load Considerations for PTFE Sliding Bearings

3. Oil Free Polymer Bearings - Fluoropolymer Formulations for Applications Needing Self-lubrication

PTFE Heat Exchangers: A Universal Solution for Corrosive Environments

The inert nature of PTFE (Teflon) has naturally endured it to applications involving corrosive chemicals. Not only does PTFE stay non-reactive to almost all chemicals (notable exceptions being sodium and alkalis at elevated temperatures), it also exhibits its properties all the way up to temperatures of 250°C. This seemingly invincible nature allows fabricators and systems designers to incorporate PTFE without worrying which chemicals may or may not present within a system.

Many chemical baths are designed to hold a host of different chemicals. The need to uniformly heat or cool these chemicals is met by the use of heat exchangers. In its most basic design, the heat exchanger consists of two adaptors, connected to one another by many lengths of tubes. The adaptors are round blocks with multiple holes through them, each meant to house one end of tube. The adaptor in turn is connected with a device that pumps out fluids, which then pass through the tube and out the other adaptor. The fluid may be heated or cooled to accordingly heat or cool the chemical bath, respectively. The tubes – which may be many meters in length – are submerged in the chemical bath, allowing for the heat transfer to take place between the chemical and the fluid within the tubes. The size, length and quantities of these tubes will define the volume of fluids than can be passed through the heat exchanger. Furthermore, the wall thickness of the tubes use will add to the efficiency of the heat transfer.

PTFE heat exchangers follow this design, with both the adaptors and the tubes being made from pure virgin PTFE. However, the fabrication process can be tricky. For one, as PTFE does not easily join even itself, the fusing of the tubes with the adaptor needs to be done in one of two ways:

1. Bonding – the outer diameter of the tube is chemically treated as are the inner holes of the adaptor. Bonding can be done using an industrial grade adhesive, which would also need to show resistance to the chemicals in the bath. The advantage of bonding is that it is much easier to fabricate such an assembly. The disadvantage is that the bond may not hold in the face of high temperatures and in the event that an unexpected chemical enters the mix

2. Welding – welding PTFE is very tricky and is as much an art as it is a science. PTFE welding can only be done if specially modified grades of PTFE are used, which allow themselves to be welding. Grades such as Chemour’s NXT and Inoflon’s M490 are examples of modified grades that can be used to make the adaptors. The tubes too need to be made accordingly. Modified grades of tubing are needed to ensure that both the tube and the adaptors are able to fuse with one another under high-temperature conditions

Aside from temperature and chemical resistance, it is also vital that the heat exchanger assembly is able to withstand the pressure build-up due to the passage of fluid. In the event of higher pressures, both the fused/bonded assembly as well as the tube itself would need to be capable of holding the same. Because of this, the wall thickness of the PTFE plays a dual role. On the one hand, because PTFE is a poor conductor, a lower wall thickness ensures that the heat transfer is more efficient. However, a thinner tube means more complications in bonding and/or welding, while also a lower capacity to withstand higher pressures.

Apart from the use of PTFE in the heat exchanger assembly, expanded PTFE (ePTFE) also finds significant use in this application. Like virgin PTFE, ePTFE also exhibits superior chemical and heat resistant properties. At the same time, the sealing properties of ePTFE ensure that it forms an effective gasket material in any portion of the assembly that may require clamping and effective sealing for the fluids.

Overall, it seems unavoidable that for certain chemical baths, PTFE is the only viable option for a heat exchanger assembly. With the proper fabrication and right materials, it offers a very effective, durable, and versatile solution for all industries where heat exchangers are needed.