Lubring (Turcite-B®) – Special Notes on Bonding and Finishing

As a globally reputed manufacturer of Lubring Slideways (Turcite-B®), we are frequently asked to provide technical assistance with regards to the bonding and finishing of the material.

Lubring is a slideway bearing material that is used primarily in machine tool building and reconditioning. As such, the machine builder is usually equipped with enough knowhow on the bonding and subsequent planing of the material, such that it forms the most effective bearing surface. However, we often supply to dealers or first time builders, who need more assistance on the bonding process.

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Preparation:

The metal surface to be mounted with Lubring Slideway can be prepared by the normal machining methods such as, grinding, milling, shaping, and planning. The surface roughness of all forms of preparation should be preferably between Ra = 1.6 µm and Ra = 3µm and not more than Ra = 6µm.

Once roughened the surfaces can be cleaned with Trichloroethylene, Perchloroethylene or Acetone. Slideway bearing material should be cleaned similarly.

Bonding:

For bonding of Lubring Slideway the following resin adhesive can be used: Ciba Geigy’s Araldite – Hardener – HV 953U; Araldite AW106. The Araldite should be applied both to metal and slideway and be spread as uniformly as possible by means of a serrated spatula. To obtain the best dispersion of the adhesive, when spreading on the slideway brush in the longitudinal direction; when spreading on the metal, brush in the transverse direction. The total quantity of bonding should be approximately 200gm per sq. mt.

Hardening:

After mounting the slideway a clamping pressure of between 0.1 – 0.3 kp/sq.cm is recommended. It is important to keep the pressure constant during the hardening process. Due to the differences in the thermal expansion coefficient of the materials, maximum curing temperature should not exceed 40°C. The hardening time for various temperatures is: 20°C min 15 hours; 25°C min 12 hours; 40°C min 5 hours.

Finishing:

After curing of the adhesive, the Lubring Slideway can be machined by conventional means. The choice depends on the machinery available viz.: grinding; grindstone.

Grinding: For grinding of Lubring Slideway use the same speed as grinding cast iron, taking care that sufficient cooling is used with an ‘open ’stone. The grindstone should be preferably silicon carbide based with rubber or polyurethane binding; grain size 80-30. Alternatively aluminum oxide with rubber bonding may also be used for soft, fine grinding action, pre-polishing and pre-mating treatment.

Oil Grooves: Lubring Slideway can be machined with oil grooves using the same methods and cutting data as used for cast iron. The form and depth of the oil grooves are optional. However, the oil grooves should never pierce through the Lubring Slideway. Oil grooves should be away from the edges by 6mm.

Metal Mating Surface: The metallic mating surface running against the Lubring Slideway should be finished to 16 Ra for optimum performance. The surface finish must never be below 14 Ra or above 20 Ra as applicable to cast iron or steel. This surface finish should be obtained by grinding in the direction of travel. Do not lap or polish to obtain this surface finish.

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The above parameters provide an effective guideline not just for Lubring, but for bonding all PTFE-related items to metal.

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Charting the standards used in defining PTFE properties

It was recently brought to our notice by an astute client that the data quoted in many of the generic online sources did not give a complete picture of the values and correct test methods needed in checking the properties of PTFE.

A quick online search of a given property of PTFE churns out a number of data sheets from various supplier websites. And although the values and standards more or less match across these sources, our own study has revealed the following:

1. Some of the standards quoted are incorrect
2. The values quoted do not have any reference as many of the standards only specify the test method and not the value reference

As a result, with an obscure polymer like PTFE, we find that information has been carried forward from older data sheets and passed on until no one is very sure what the “correct” value is anymore. We ourselves have reached a dead-end on a number of metrics, but we have done our best to fill the gaps using verifiable data.

Let’s look at point (1) above. The most commonly quoted standard for PTFE is ASTM D 1457. We see this standard in a number of places and only after trying to buy a copy online were we informed that ASTM D 4894 had replaced the ASTM D 1457 in 2001.

Clients who – due to the effect of legacy – still refer to the ASTM D 1457 sometimes get upset when we send them test reports quoting ASTM D 4894 and it requires some discussion with their QA team before the new standard is accepted.

However, even the ASTM D 4894 only applies to virgin PTFE. For filled grades of PTFE, we refer to the ASTM D 4745. This again requires a discussion with the client as is especially problematic when the client orders a very specialized grade. Since the ASTM D 4745 only covers the more general filled grades of PTFE, clients who order an irregular grade feel frustrated that there is no standard pertaining to their requirement.

Both the standards, however, do provide some basic values of tensile strength, elongation and specific gravity, which help in checking whether the properties attained after testing are in line with the requirements.

However, as the table below shows, very few of the standards actually give any values. For a whole list of properties, the standards only tell you how to check the value, but do not make any recommendations on what those values should be. Furthermore, due to PTFEbeing a niche polymer, some of the standards – such as ASTM D 2240 – actually pertain to other materials and the test method is simply employed for PTFE.

	Virgin		Filled grades
	Standard	Value in standard	Standard	Value in standard
Density	ASTM D 4894	Yes	ASTM D 4745	Yes
Avg. Particle Size	ASTM D 4894	Yes	ASTM D 4894	Yes
Tensile Strength	ASTM D 4894	Yes	ASTM D 4745	Yes
Elongation at break	ASTM D 4894	Yes	ASTM D 4745	Yes
Shore D Hardness	ASTM D 2240	No	ASTM D 2240	No
Linear Expansion Coefficient (-50°C to +15°C)	ASTM E 831 / ASTM D 696	Yes (696)	ASTM E 831 / ASTM D 696	No
E-modulus (tensile)	ISO R 527	No	ISO R 527	No
Wear resistance (with Taber abraser method)	ASTM G 195-08	No	ASTM G 195-08	No
Deformation Under Load – Total deformation after 24H	ASTM D 621 A	No	ASTM D 621 A	No
Static Coefficient of friction	ASTM D 1894	No	ASTM D 1894	No
Dielectric Strength	ASTM D 149	No	ASTM D 149	No
Dieelectric Constant	ASTM D 150	No	ASTM D 150	No

As mentioned earlier, my client was curious to know what benchmarks were being followed when we quoted the values expected for each metric. However, we were unable to find any organisation that actively published data on PTFE and its filled grades.

In trying to trace back the values seen across so many data sheets (they are all in the same range, so we assumed they have some common source), we were able to find references old manuals released by DuPont, Dyneon and Daikin from where these values were obtained. Obviously, once we referenced DuPont, the client was satisfied and we were able to neatly define both the correct standard and the value with the proper reference.

It is however interesting to note that as widely used and accepted as PTFE is, there still does not exist any up-to-date properties standard that can be used by manufacturers as reference. The DuPont website does have values of virgin PTFE – but they reference the ASTM D 1457 – which suggests that maybe the information is dated. Not to say that the values would have changed significantly, but QA is a continuous process and something published within the last decade might offer a lot of support to both manufacturers and OEMs alike.

Demystifying Rulon

We have earlier looked at Turcite B* and explained how it is a result of a very successful branding exercise that has stood the test of time. In truth, as we now know, Turcite* is a PTFE based composition and has been successfully substituted in many applications with equivalent PTFE formulations.

Another very successful branding venture has been that of Rulon*. Although we do not see the demand for Rulon* being as high as that of Turcite*, there has been a very conscious and well thought out strategy which has kept the compositions of this brand ambiguous, to the point that clients find it very tough to accept any alternatives.

In addition to this, the unique pigmenting of each Rulon* grade offers further ambiguity. Visually, a client is unable to reconcile with a substitute when the colors do not match. It should be mentioned here that in many cases, we have seen that pigments help alter the properties of PTFE in a very measurable and positive way. For example, the green-blue pigment of Turcite* has been proven to offer better PV values than the same composition in, say brown color. Therefore, while the pigmenting of Rulon* does help the branding considerably, we would assume the pigments themselves were not chosen randomly, but by testing different variants and choosing the one that had maximum impact on the properties required.

We have done some research to try and lay out the compositions of the most popular Rulon* grades, in the hope that it will make the choice a little easier for an OEM or manufacturer. In most cases, these appear to be regular PTFE grades that have been made unique using pigments. In some cases, such as Rulon J*, the grade is not regular, but can be easily blended as long as one knows the composition. The table below shows the various compositions and attributes of the most common grades of Rulon*.

Product Description	Filler Details	Max Load (Mpa)	Max. PV ((psi-fpm); Mpa-m/s)	Properties	Colour
Rulon LR	PTFE+15% Glass	6.9	10,000; 0.35	High creep and abrasion resistance	Maroon
Rulon AR	PTFE+25% Glass	6.9	10,000; 0.35	Wear resistant, improved hardness, lower thermal expansion, lower deformation under load	Maroon
Rulon 142	PTFE+Bronze (40-60%)	6.9	10,000; 0.35	High thermal conductivity; better creep resistance; linear bearing material	Turquoise
Rulon 641	PTFE+15% Mineral	6.9	10,000; 0.35	Used mainly in food processing, FDA approved	White
Rulon J	PTFE+15% Polyimide	5.2	7500; 0.26	Good friction against soft metals	Gold

We would like to point out a few things pertaining to the values of this table:

1. The Load values of Rulon* across grades seem to be considerably lower than those of comparable regular grades of PTFE. For example, PTFE+15% Glass has a tensile strength of >20 Mpa when tested in-house – which is almost 3 times what Rulon* offers. The reason for this lowering of load metrics is not quite known. Most likely the addition of pigments causes some sacrificing of load values
2. The PV values are comparable with regular grades of PTFE, however not so vastly different that it makes Rulon* superior in any obvious way. For example, Rulon LR* offers a PV of 10000, whereas PTFE+15% Glass offers only 7500. However, Rulon AR* also offers a PV of 10,000, whereas PTFE+25% Glass offers 12,000.

In a nutshell, we do not believe that the uniqueness of Rulon* pertains to any significant improvement in properties, but to a branding push given when PTFE was still an ambiguous material for many buyers. In recent times, many clients have adopted substitutes as they rightly feel the premium attached to Rulon* material is unjustified. Although rigorous testing is first done to prove that the substitute matches up with Rulon*, we have found that regular materials are more than equal to the task.

* Rulon is a brand name of Saint-Gobain Plastics; Turcite is a brand name of Trelleborg Sealing Solutions

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 in 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 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 a 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 seal 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 seals. 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.

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 1/4^th 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 1/8^th 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