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

PCTFE vs PTFE - A Comparison of Two Very Similar Polymers

Even though PTFE remains a niche polymer among more generic materials such as PP (Polypropylene), PVC, PE (Polyethylenes, such as HDPE and LDPE), and even Nylons, within the engineering space it is now quite common. Most applications involving high temperature, corrosive chemicals, high voltages, or high wear/friction now look to PTFE automatically as a solution. 

Despite this, there do exist applications where PTFE does not fit the bill and a compromise must be made. For example, applications where high dimensional stability is needed across a wide temperature range, PTFE tends to fall short. The high linear thermal expansion coefficient of PTFE means that it cannot hold its dimensions as temperatures vary. In such a situation, we have seen PEEK being adopted. While PEEK does do the trick, it is also 10X the cost of PTFE.

Similarly, certain applications where cost is a constraint need to make do with POM (Delrin), or even PVC, where PTFE cannot be used. In such a scenario, we possibly forego some of PTFE’s key properties.

Over the years a variety of new polymers that have been developed to fill the performance and commercial gaps between PEEK and PTFE. These include PFA, FEP, PEK, PPS (Ryton), and PCTFE.

Although not well known, PCTFE forms an ideal substitute for PTFE in certain applications where PTFE is unable to perform adequately. The table below is meant to offer a snapshot comparison of the two, such that any application engineer can evaluate the key differences.

 

 

Unit

PTFE

PCTFE

Remarks

Properties

Tensile Strength

Mpa

20-30

30-35

With a marginally higher tensile strength, PCTFE rates higher than PTFE on this metric

Elongation

%

200-350

100-250

PCTFE is stiffer than PTFE, which means it lacks some of the softness of PTFE when it comes to sealing, but that it also holds its dimensions more easily

Melting Point

Deg. C

350-380

200-220

PTFE is still preferred on outright high-temperature applications

Dielectric Breakdown Voltage

KV/mm

50-100

20-40

PTFE rates higher on outright dielectric strength

Coefficient of Friction

 

 0.03-0.05

0.25-0.35 

PTFE rates higher as a non-stick material 

 

 

 

 

 

 

Processing

Injection Moulding

 

No

Yes

PCTFE has more versatility when processing, allowing for more complex parts

Compression Moulding

 

Yes

Yes

 

 

 

 

 

 

Characteristics

Chemical Resistance

 

Extreme

Very Good

PTFE is still unmatched in chemical resistance

Thermal Stability

 

OK

Very Good

PCTFE rates higher than PTFE when it is a question of stability over a wide range of temperatures

Price

 

Med

High

PCTFE is more expensive than PTFE, and is therefore used in specific applications only

As you can see from the above chart, PCTFE and PTFE each have unique advantages and disadvantages when compared with one another. Like all polymers, the application needs to be properly understood and the commercials need to be weighed in before any decision can be made.

However, it is fair to say that when dimensional stability across a temperature range is a must, PCTFE is growing to become the most effective substitute for PTFE.

Datasheet for PCTFE:

Property

Value

Units

Method

MECHANICAL PROPERTIES

Tensile Strength

4860 - 5710
34 - 39

psi
MPa

D 638

Elongation

100 - 250

%

D 638

Flexural Strength, 73°F

9570 - 10300
66 - 71

psi
MPa

 

Flex Modulus

200 – 243 x 103
1.4 – 1.7

psi
MPa

 

Impact Strength, Izod, 23 deg C

2.5 – 3.5

ft-lb/in

D 256

Compressive Stress at 1% deformation,

1570 – 1860
11 - 13

psi
MPa

D 695

Density

2.10 to 2.17

gm/cu.cm

 

THERMAL PROPERTIES

Coefficient of Linear Expansion

7 x 10-5

K-1

 

Melting Point

410 -414
210 - 212

deg F
deg C

 

Thermal Conductivity

1.45
0.84

Btu·in/h·ft2·°F
W/m·K

ASTM C 177

Specific Heat

0.22
0.92

Btu/lb/deg F
kJ/Kg/deg K

 

Heat Distortion Temperature, 66 lb/sq.in (0.455 MPa)

259
126

deg F
deg C

D 648

Processing Temperature

620
327

deg F
deg C

 

ELECTRICAL PROPERTIES

Dielectric Strength, short time, 0.004”

3000

Volt/mil

D 149

Arc-Resistance

360

sec

D 495

Volume Resistivity, @ 50% RH

2 x 1017

ohm-cm

D 257

Surface Resistivity, @ 100% RH

1 x 1015

Ohm sq-1

D 257

Dielectric Constant, 1 kHz

2.6

ε

D150-81

Dissipation Factor, @ 1 kHz

0.02

 

D150-81

OTHER PROPERTIES

Water Absorption

0.00

% increase in weight

D570-81

Flame Rating+

Non-flammable

 

D 635

Coefficient of friction (Dynamic)

 

0.25-0.35

D 1894

Specific Gravity

2.10 to 2.17

 

D792

Moisture Permeability Constant

0.2

g/m, 24 hours

 

O2 Permeability

1.5 x 10-10

Cc, cm/sq.cm, sec, atm

 

N2 Permeability

0.18 x 10-10

Cc, cm/sq.cm, sec, atm

 

CO2 Permeability

2.9 x 10-10

Cc, cm/sq.cm, sec, atm

 

H2 Permeability

56.4 x 10-10

Cc, cm/sq.cm, sec, atm