Fluoric polymers (PTFE/PVDF)

Fluoropolymers have excellent, almost universal chemical resistance. They can be used both at high and very low temperatures (-260 to +260°C). They also possess outstanding resistance to weathering (UV-stability).
Due to the low coefficient of friction, they are often used as sliding materials or as corresponding additives in other high-performance plastics.

TECAFLON PVDF (PVDF)

Polyvinylidenefluoride (PVDF) – TECAFLON PVDF – is an opaque, semi-crystalline, thermoplastic fluoropolymer. PVDF is characterized by excellent chemical stability, without having the disadvantages of low-mechanical values and/or processing difficulties which can be experienced with other fluoroplastics.
  • low density compared to other fluoropolymers
    good mechanical strength compared to other fluoropolymers
    high permanent operating temperature (140°C)
    practically no moisture absorption
    good dimensional stability
    high chemical resistance
    good resistance to hydrolysis
    weather-proof
    radiation resistant
    good electrical insulator
    high abrasion resistance

TECAFLON PTFE natural (PTFE)

Polytetrafluoroethylene (PTFE) – TECAFLON PTFE natural – is a semi-crystalline fluoropolymer with an unusually high chemical and thermal resistance (-200 to +260°C, temporarily up to 300°C). In addition, this thermoplastic material has excellent sliding properties, a non-stick surface and the best insulating properties. This is in contrast, however, to low mechanical strength and a high specific gravity, compared to other plastics. In order to improve the mechanical properties, PTFE can be used as a compound reinforced with additives such as glass fibre, carbon or bronze. Due to its structure, this material is formed into semi-finished products using a compression processes and machined afterwards with cutting/machining tools.
  • extremely high chemical resistance
    very good temperature resistance (-200-+260°C, temporarily up to 300°C)
    very low coefficient of friction (static friction = sliding friction)
    extremely low surface tension (practically no materials stick to PTFE -> difficult to adhere to or weld)
    high coefficient of thermal expansion
    relatively low strength/rigidity
    low dielectric constant
    non-combustible

 

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Polymer Particle Structure

The particles of polymer produced in the dispersion polymerisation process are of the order of 0.2µm in size, whilst those from a granular polymerisation are hundreds of µm in size, built up from smaller particles.They are both highly crystalline – about 90 to 95%.

The dispersion particles can be studied directly in a conventional transmission electron microscope, providing the electron intensity is kept low. On raising the electron beam intensity the particles change rapidly in appearance to become transparent with a crumpled texture. At this stage the crystallinity has disappeared and the ‘particles’ probably consist of a shell of carbon. Electron diffraction patterns and dark field micrographs suggest that the particles are composed of a pile of small single crystals with the molecular axis along the axis of the brick-shaped particles . The particles also appear to have a striated surface structure generally parallel to the long axis. A replica of some dispersion particles. During coagulation, the dispersion particles aggregate to form a larger particle, made up of a loose structure of agglomerates of the primary particles. During cold, lubricated extrusion the agglomerated particles are highly distorted, with their primary particles becoming aligned and also drawn into fibrous material.

During the early stages of polymerisation granular particles form as aggregates of smaller particles. This process continues and large irregular fibrous structures are produced.This material is then modified mechanically to reduce it to the familiar form suitable for processing.

Porous PTFE Plastic Polymer

Polytetrafluoroethylene (PTFE) is a versatile polymer used in a broad range of medical, industrial, and high technology applications. Porous PTFE is inert, making it unreactive to most chemicals, and, it can work in high processing and operating temperature environments (500°F / 260°C continuous).

Porex porous PTFE offers excellent thermal, electrical, mechanical, and chemical properties as well as superior control over pore size, permeability, water entry pressure, and thickness. PTFE is an ideal choice for a wide variety of demanding applications requiring one or a combination of these exceptional properties.

PTFE MANUFACTURING STANDARDS

PTFE materials are manufactured in Class 100k clean rooms under ISO 9001:2008 quality standards. They are available in a variety of formats including rolls, sheets, die cut, and rotary cut parts and also with adhesive backings.

KEY BENEFITS

  • chemically inert
  • thermally resistant (500°F / 260°C continuous use)
  • compatible with ultrasonic/thermal welding and inset molding
  • hydrophobic and oleophobic
  • omni-directional flow
  • high tensile strength
  • steam and EtO (ethylene oxide) sterilizable
  • natural white or black color
  • UL 94 V-0 @ 0.1mm and 5VA @ 0.75mm
  • UL 746C f1 rating (outdoor weatherability)
  • low dielectric constant (to 1.43)
  • diffuse reflectivity up to 98%
  • used in medical devices

 

About PTFE’s Physical Properties

The structure of PTFE molecules
PTFE, poly(tetrafluoroethene), is made by polymerising lots of tetrafluoroethene molecules.
PTFE's Physical Properties
This simple diagram for PTFE doesn’t show the 3-dimensional structure of the molecule. In the simpler molecule poly(ethene) the carbon backbone of the molecule just has hydrogen atoms attached to it, and the chain is very flexible – it definitely isn’t a straight molecule.
However, in PTFE, the fluorine atoms in one CF2 group are big enough to interfere with those on the neighbouring groups. You need to remember that each fluorine atom will have 3 lone pairs sticking out from it.
The effect of this is to inhibit rotation about the carbon-carbon single bonds. The fluorine atoms will tend to line up so that they are as far apart as possible from neighbouring fluorines. Rotation will tend to involve a clash of lone pairs between fluorines on adjacent carbon atoms – and this makes rotation energetically unfavourable.
The repulsions lock the molecules into a rod-like shape with the fluorines arranged into very gentle spirals – a helical arrangement of the fluorines around the carbon backbone. The rods will then tend to pack together a bit like long thin pencils in a box.
This closely touching arrangement has an important effect on the intermolecular forces as you will see.
Intermolecular forces and the melting point of PTFE
The melting point of PTFE is quoted as 327°C. That’s quite high for a polymer of this sort – so there must be sizeable van der Waals forces between the molecules.
But . . . several web sites talk about PTFE having very weak van der Waals forces. If it had very weak van der Waals forces, it would be a gas – not a fairly high melting point solid. So we have a problem here!
Why do people claim the van der Waals forces in PTFE are weak?
van der Waals dispersion forces are caused by temporary fluctuating dipoles set up as electrons in the molecules move around. Since PTFE molecules are large, you would expect the dispersion forces to be large as well, because there are a lot of electrons which can move.
It is generally the case that the bigger the molecule, the greater the dispersion forces.
However, there is a problem with PTFE. Fluorine is so electronegative that it tends to hold the electrons in the carbon-fluorine bonds closely to itself – so closely that the electrons are prevented from moving as much as you would expect. We describe the carbon-fluorine bonds as not being very polarisable.
van der Waals forces also include dipole-dipole interactions. But in PTFE each molecule is sheathed in a layer of slightly negative fluorine atoms. The only interactions possible between molecules in this case are repulsions!
So the dispersion forces are weaker than you might expect, and dipole-dipole interactions are going to tend to cause repulsion. It is no wonder that people claim that van der Waals forces are weak in PTFE. You don’t actually get repulsion because the effect of the dispersion forces outweighs that of the dipole-dipole interactions, but the net effect is that the van der Waals forces will tend to be weak.
And yet PTFE has a high melting point, and so the forces holding the molecules together must be strong.
How can PTFE have a high melting point?
PTFE is very crystalline in the sense that there are large areas where the molecules are lying in a very regular arrangement. Remember that PTFE molecules can be thought of as long thin rods. These rods will pack very closely together.
That means that although PTFE molecules can’t generate really big temporary dipoles, the dipoles that are produced can be used extremely effectively.
So are the van der Waals forces in PTFE weak or strong?
I think you could argue it both ways! If you had PTFE chains arranged in such a way that the chains didn’t have much close contact, then the forces between them would be weak, and the melting point would be low.
But in the real world, the molecules are closely touching. The van der Waals forces may not be as strong as they could be, but the structure of the PTFE means that they are felt to the maximum effect, producing overall strong intermolecular bonding and a high melting point.
Non-stick properties and friction
Virtually every site that I have looked at treats the relative lack of friction of PTFE and its non-stick properties as if they were the same effect. I don’t think that’s true.
The non-stick properties
This is about why things like water and oil don’t stick to the surface of PTFE, and why you can fry an egg in a PTFE-coated pan without lots of it ending up stuck to the pan.
You need to consider what forces might hold other molecules to the PTFE surface. Possibilities might include some sort of chemical bonding, van der Waals forces or hydrogen bonds.
Chemical bonding
Carbon-fluorine bonds are very strong, and there is no way that any other molecule could get at the carbon chain to enable any sort of substitution reaction to take place. No sort of chemical bonding could take place.
van der Waals forces
We’ve seen that the van der Waals forces in PTFE aren’t very strong, and only work to give PTFE a high melting point because the molecules lie so close together and there is very effective contact between them.
But it is different for other molecules approaching the surface of the PTFE. A relatively small molecule (like a water or an oil molecule) will only have a small amount of contact with the surface, and will only produce a small amount of van der Waals attraction.
A large molecule (like a protein, for example) isn’t going to be rod-like and so, again, there isn’t going to be enough effective contact between it and the surface to overcome the low tendency of the PTFE to polarise.
Either way, van der Waals forces between the PTFE surface and whatever is around it are going to be small and ineffective.
Hydrogen bonds
The PTFE molecules on its surface are completely encased in fluorine atoms. Those fluorine atoms are very electronegative and so will all carry some degree of negative charge. Each fluorine also has three lone pairs of electrons sticking out.
Those are exactly the conditions needed for hydrogen bonding to be possible between lone pairs on the fluorines and hydrogen atoms in water for example. But it clearly doesn’t happen – otherwise there would be strong attractions between PTFE molecules and water molecules and water would stick to the PTFE.
In November 2013, an Iranian PhD student pointed out to me a 1997 paper by Dunitz and Taylor with a title “Organic Fluorine Hardly Ever Accepts Hydrogen Bonds”. If you are interested, you can find it from this site if you have the right access.
They found that only a tiny number of compounds containing C-F bonds would form hydrogen bonds, whereas compounds like HF or the F- ion formed strong hydrogen bonds.
What they didn’t come up with, however, was any definite explanation for this, although they suggested that a possible explanation could lie in the fact that the fluorine atom holds its electrons very tightly in towards the nucleus, and as a result the C-F bond isn’t very polarisable. The electrons won’t move sufficiently towards a hydrogen from water (or anything similar) in order for a hydrogen bond to form.
Personally, I have a problem seeing why that is different from the situation in H-F or a fluoride ion, both of which can form hydrogen bonds with water.
And their final sentence said:
“At the same time, it has to be admitted that, in spite of the vast amount of work on hydrogen bonding over the years, the chemical factors influencing the strength of hydrogen bonds (especially factors influencing H-bonding acceptor ability) are still not completely understood.”
Summary
There are no available methods for other molecules to attach themselves successfully to the surface of the PTFE, and so it is has a non-stick surface.
The low friction
PTFE has a very low coefficient of friction. What this means is that if you have a surface coated with PTFE, other things will slide on it very easily.
What follows is just a quick summary of what is happening. This comes from a 1992 paper called Friction and wear of PTFE – a review which is available free from this link.
At the start of sliding, the surface of the PTFE fractures, and lumps are transfered to whatever it is sliding against. That means that the PTFE surface tends to wear away.
As sliding continues, the lumps are spread out to a thin film.
At the same time the surface of the PTFE is dragged out into an organised layer.
The two surfaces in contact now both have well organised PTFE molecules which can slide over each other.
What holds the PTFE layer onto the substance it is sliding against is quite complicated, and thoughts on this may have changed since the paper was written. If you are interested, you will find it discussed on the page numbered as 203 of the paper (page 11 of 19 on my pdf reader).

Material Comparison – PVC & PTFE

PTFE‘s significant chemical, temperature, moisture, and electrical resistances make it an ideal material whenever products, tools, and components need to be durable and reliable in even the most strenuous applications. On top of this, PTFE coated wire boasts unique low-temperature durability and fire resistance that make it a good choice for a constantly growing list of products, components, and applications.

PVC & PTFE

PVC cables are manufactured at a much lower cost because they don’t have to meet the high temperature rating that PTFE cables do. PVC stands for Polyvinyl Chloride which is a compound engineered to be used in electronics and other indoor applications.

PVC electronic cables meet a 105°C temperature rating in 300 or 600 volts. They’re manufactured with flexible copper strands and PVC insulation on the inner conductors. They sometimes come with an aluminum overall foil shield and a tinned copper drain wire.

Now that you know the construction of a PVC cable you’ll probably understand why a PTFE cable is much more expensive. PTFE compound is used to manufacture a 200°C temperature material to cover the copper conductors. The engineering involved in creating a 200°C cable is much more involved than the process of creating a 105°C cable.

Property Name Units   ABS/PVC Alloy    PTFE Filled
Specific Heat Capacity BTU/lb-°F 0.382
Shear Modulus ksi
Compressive Yield Strength psi 3050 5710
Poisson’s Ratio
Dielectric Constant 3.3 3.7
Dielectric Strength kV/in 508 467
Elongation at Break % 100 9.4
Flexural Yield Strength psi 7030 9820
Modulus of Elasticity ksi 319 348
Hardness, Rockwell R 88 110
Tensile Strength, Ultimate psi 4030 6580
Tensile Strength, Yield psi 5420 8270
Electrical Resistivity ohm-cm 1.00e+14 3.00e+15
Density lb/in³ 0.0423 0.0531
Max Service Temperature, Air °F 170 212
Thermal Conductivity BTU-in/hr-ft²-°F 1.87 1.67
Fracture Toughness ksi-in½