Friday, January 13, 2012

UHMWPE - the unknown polymer


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One of the few good things to happen due to the unprecedented escalation of PTFE prices globally was that it allowed us to look at alternate materials and seriously gauge the feasibility of manufacturing them.

In an earlier post, we looked at the various properties of PTFE and compared them to the other polymers. And although the key takeaway from that exercise was that PTFE was an immensely versatile material which was difficult to replace, we did make mention of possible alternatives, provided the user was willing to compromise on some parameters.

A key polymer which struck us then and continues to feature prominently in our product offering today is UHMWPE. We would like to take a more detailed look at UHMWPE for 2 reasons:



  1. It does measure up against PTFE as a low-cost substitute (with certain limitations)
  2. It’s properties do not seem to be as widely known to end-users, resulting in limited use in many applications where it would otherwise be ideal

What is UHMWPE?

Sometimes referred to as just “UHMW”, UHMWPE or Ultra High Molecular Weight Poly Ethylene is an off-white polymer that exhibits superior strength while being both light-weight and possessing a low coefficient of friction.

While it is not entirely accurate to refer to it as an “unknown” polymer – our own analysis of search terms within Google tells us that a total of ~62,000 searches per month are done globally for UHMWPE and/or UHMW. This is tiny in comparison to searches for PTFE/Teflon (1,300,000 per month) or for Nylon (5,500,000 per month).


Comparison with PTFE

So how does UHMWPE compare with PTFE? In our own opinion – it compares rather well. In fact, if you take all the applications involving PTFE and remove the ones that call for heat resistance, UHMWPE is a very workable substitute.

Although a full comparison chart is given at the end of this article, we would like to look at some specific properties more subjectively.



  1. Temperature resistance
    Let’s get this one out of the way, since we know that it is UHMWPE’s weakness. Having an operating temperature of only about 80°C compared with 260°C for PTFE,
    UHMWPE is automatically disqualified in a range of industrial applications where the temperatures surrounding the material are expected to be well in excess of it’s upper limit.
  2. Wear resistance
    Before we were familiar with
    UHMWPE, we were asked to advice a cement plant on whether they could use Lubring sheets (PTFE+Bronze) in a wear application. We were confident that it would work and when they mentioned that they had tried UHMWPE and it had failed, we did not think it was worth looking into. But when we did compare the materials, we realized that if UHMWPE had failed, there was little chance PTFE would work – since the gap between the two materials on this parameter is quite wide.
    Keep in mind that
    PTFE+Bronze is the most wear resistance grade of PTFE available. So if we compare UHMWPE with plain PTFE, the rift is even wider.
  3. Coefficient of friction
    It is difficult to beat PTFE on this parameter, although
    UHMWPE comes fairly close. While it remains true that the coefficient of friction between PTFE and polished stainless steel is the lowest between two known solids (0.03-0.05), UHMWPE is able to reach a somewhat respectable 0.1-0.15 on this metric. While this does put it out of range for many applications where the recommended coefficient cannot exceed 0.1 (eg: sliding bearings) – it is a useful substitute in components where smooth movement between parts is the only requirement.
  4. Dielectric strength
    Both materials are pretty much neck and neck on dielectric strength. Where
    UHMWPE loses out is on its ability to be skived into thin tapes. While we regularly skive PTFE down to 0.04-0.05mm thicknesses, the same is more challenging with UHMWPE, since it lends a much higher wear on to the skiving blade, making it difficult to achieve long lengths of tape before the blade dulls out and breaks the tape. Nonetheless, thicknesses of 0.1mm and above are more than feasible, meaning that as an insulating pad or even a component used in high voltage applications, UHMWPE is more that suitable.
  5. Chemical inertness
    PTFE is well known for it’s inertness and this allows it to lend itself to applications ranging from biotechnology to medical devices and chemical linings. While UHMWPE does not have quite the same extreme inertness as PTFE, it does find use in medical applications (it is used in parts for joint replacements) and can easily be used in both biotech and chemical applications, provided the exact nature of chemicals is known and compared against it’s capabilities.
  6. Weight
    While weight has never been a consideration for
    PTFE in any of it’s applications, we would still like to highlight that UHMWPE is less than half the weight of PTFE (specific gravity of 0.95 vs. 2.15 for PTFE). The key difference this adds is in their respective cost cacluations. Not only is UHMWPE cheaper in resin form (roughly 1/4th the cost per Kg), the fact that you consume only half the weight to get the same volume part implies that the effective cost is 1/8th the cost of PTFE. This represents a significant saving.
So where can we use UHMWPE?

There are a range of applications where UHMWPE could and should be used. In many cases, we have tried to suggest to the end-user that we can offer them UHMWPE in stead of PTFE, but due to restrictions on standards and because changing specifications can be time consuming, very few have opted for the change.

Strangely, in many cases, clients have opted for suppliers offering reprocessed PTFE, but not UHMWPE. Given the highly diminished properties of reprocessed PTFE, this is functionally not a great trade-off in the medium to long term.

Automotives

Most automotive applications use PTFE in high temperature environments, so UHMWPE does not fit the requirement. However, there are a number of applications where the parts operate at room temperature eg: car doors, seats, hand levers etc. and here UHMWPE can find a lot of use. We are aware that the wear strip used inside car doors employs UHMWPE.

In general, UHMWPE wear strips offer a low cost and effective alternative to PTFE wear strips.

Valves and seals

Typically, valves and seals require a low coefficient of friction with a good wear resistance.  UHMWPE is an excellent replacement for PTFE in these areas.

Medical

UHMWPE is widely used in joint replacements due to its chemical inertness and light-weight.

Infrastructure

Although regulatory restrictions prevent materials other than PTFE to be used POT bearings, there are many sliding bearing applications which do not fall under the government codes and are therefore potential areas where UHMWPE can be used. UHMWPE could be employed successfully in sliding bearings and as plain sliding pads.

Electronics

Many components used in electronics have traditionally employed PTFE components for insulation. In a number of cases, we have successfully tested UHMWPE for these applications and convinced the client to shift.


Overall, there continues to be a resistance to employ a material like UHMWPE. Part of this is regulatory – drawings and specifications that call for PTFE cannot be changed over night. But mostly there is a genuine dearth of awareness about the material – which is equally difficult to change. While it is true that UHMWPE is a substitute for PTFE – we see it as more of a partner in application – allowing many end-users to find a competitive, low-cost solution where they would otherwise be unable to proceed with their development or manufacturing.


Comparison chart between PTFE and UHMWPE


UHMWPE
PTFE
Units
Colour
Off-white
White

Specific Gravity, 73°F
0.944
2.25

Tensile Strength @ Yield, 73°F
3250
4000
psi
Tensile Modulus of Elasticity, 73°F
155,900
150,000
psi
Tensile Elongation (at break), 73°F
330
350
%
Flexural Modulus of Elasticity
107,900
145,000
psi
Compressive Strength at 2% deformation
400
1650
psi
Compressive Strength 10% Deformation
1200
2200
psi
Deformation Under Load
6-8%
2.5-5%
%
Compressive Modulus of Elasticity, 73°F
69,650
79,750
psi
Hardness, Durometer (Shore "D" scale)
69
55-65

Izod Impact, Notched @ 73°F
30
161
ft.lbs./in. of notch
Coefficient of Friction (Dry vs Steel) Static
0.17
.06-0.12

Coefficient of Friction (Dry vs Steel) Dynamic
0.14
0.12

Sand Wheel Wear/Abrasion Test
95
90
UHMW=100
Coefficient of Linear Thermal Expansion
11
6-7.2
in/in/°F x 10-5
Melting Point (Crystalline Peak)
135-145
380
°C
Maximum Service Temperature
80
260
°C
Volume Resistivity
>1015
NA
ohm-cm
Surface Resistivity
>1015
NA
ohm-cm
Water Absorption, Immersion 24 Hours
Nil
Nil
%
Water Absorption, Immersion Saturation
Nil
Nil
%
Machine-ability Rating
5
3
1 = easy, 10 = difficult



3 comments:


  1. UHMWPE Polymer


    It’s going to be ending of mine day, but before ending I am reading this great piece of writing to increase my knowledge.

    ReplyDelete
  2. PTFE seals and coatings are important in the pharmaceutical industry because they provide over braid seals and strength and ease when cleaning and purging the system.

    ReplyDelete
  3. This sounds great, really spreading the word. You've shared some important ideas here.

    ReplyDelete