GCSE and A level chemistry exam revision notes on smart materials and some high performance polymer materials

Part 6 GORE-TEX  ®, THINSULATE and TEFLON-PTFE - structure, properties and uses

Part 6a. GORE-TEX

• Nylon is a synthetic polymer used to make fabrics for the clothing industry, but on its own it is not very waterproof.
• If the nylon is coated with polyurethane it becomes a much tougher durable waterproofed material for outdoor clothing.
• Unfortunately the polyurethane coating doesn't allow water vapour through so you get too hot, sweaty and uncomfortable, because the sweat can't evaporate to keep you cool.
• Gore-Tex is the registered trade name often applied to a particular design of (usually outdoor) clothing fabric that is designed to keep you dry in the rain without getting sweaty.
  • Strictly speaking it is the patented name a porous form of the polymer PTFE made by stretching the polymer fibres in a controlled way to create fine pores.
  • So the GoreTex itself is actually one layer in the fabric design for a particular clothing application e.g. anorak or wetsuit.
• GORE-TEX ® is an excellent breathable, waterproof textile fabric used in high-performance clothes such as walking boots, hiking boots and mountain coats. Unlike ordinary synthetic textiles like nylon, GORE-TEX stops rain from getting in but lets perspiration (sweat condensation) out. So it keeps you dry on the outside and dry on the inside at the same time.
• BUT how does a Gore-Tex  membrane work?
• GoreTex fabric is made up of a layer of a plastic based on expanded PTFE and this is laminated onto a layer of another fabric. Gore-Tex is a laminate containing layers of the polymers nylon, PTFE (Teflon) and polyurethane.
  • The PTFE layer contains millions of tiny holes (called pores) per square millimetre. The pores are 20,000 times smaller than a water droplet, BUT, individual water molecules are 700 x smaller than pore, so water molecules can pass through the pores, but NOT water droplets e.g. of rain!
  • Each pore is big enough to allow individual water molecules to pass through, BUT, not big enough for water droplets to pass through. The intermolecular bonding forces between water molecules are quite strong and water droplets like to stay intact due to these attractive forces rather than adhere to the hydrophobic PTFE layer.
  • It is the tiny holes in the PTFE layer that let individual water vapour molecules through, i.e. it is breathable, so sweat escapes through the micro pores but rain water (liquid droplets) can't get in.
  • The outer fabric surface repels water - hydrophobic in nature, combined with the tiny size of the PTFE pores ensures the water-proof properties of Gore-Tex fabrics.
  • 
• Each pore is too small for water droplets to pass through, but big enough to let water vapour molecules from sweat to go through (transpiration or 'breathability').
  • This illustrated in the diagram above, the fabric material is multi-layered (a sort of 'sandwich' composite).
  • So, if you sweat in this 'breathable' material, the water vapour can escape keeping you cooler, and you don't get the discomfort from sweat condensate,
    • AND, because water droplets can't pass through the outer tough protective layer, you should keep dry in wet weather.
  • Also note, that without the outer tough protective layer e.g. nylon or polyester, the GoreTex membrane would be easily damaged and it wouldn't be waterproof and windproof.
  • Gore-Tex products have all the useful properties of nylon-polyurethane fabrics but breathable materials!
• Therefore you have a 'breathable' fabric which can be combined with insulation materials such as Thinsulate to make outdoor clothing that keeps you both dry (waterproof) and warm (insulating).

• -

Part 6c. THINSULATE

• THINSULATE is the trademark of a specially designed commercial synthetic fibre material that can be used for insulation in clothing e.g. gloves or winter jackets.

• Thinsulate fibres are much thinner than most other artificial fibres like nylon an polyester used in clothing.

• Human hair is 0.1 mm (100 μm), thinsulate fibs 0.010 - 0.015 mm, 10-15x thinner!

• molecule same as Lycra?

• They are produced as a dense tangled web of fibres that trap lots of air between the thin fibres and reflect heat too.

• Polyesters like polyethylene terephthalate (PET), poly(propene) and poly(ethene) are used to make Thinsulate.

• This considerably reduces heat loss, i.e. from your body and so clothes containing layers of this material keep you very warm.

  • They are supposed to be at least as good as duck down for insulation, but less water-absorbent and resistant to crushing than duck down.

  • The good thermal insulation effect is due to the high density of fibres produced by utilising very fine fibres which increases the efficiency of the air gaps to reduce heat flow.

  • Thinsulate is supposed to allow moisture to escape e.g. from sweat, which is supposed to evaporate, so Thinsulate is credited in being a 'breathable' material like Gore-Tex. described above.

• A further advantage is that thinsulate fibres absorb very little water, so they retain their insulating properties and still keep you warm even if your clothes get wet.

• House insulation

Part 6c. TEFLON - poly(tetrafluoroethene)

TEFLON need ref in oil-polymer pages

• The formation of the polymer of poly(tetrafluoroethene) by the polymerization of the monomer tetrafluoroethene (n is a very large number and can be several thousand!)

• TEFLON is the trade name the polymer called poly(tetrafluoroethene) or polytetrafluoroethene, known by its acronym of PTFE.

• PTFE is noted for its marked chemical inertness and high thermal stability - very heat resistant, only softens at 320^oC.

  • The excellent chemical resistance is characteristic of all highly fluorinated polymers.

  • It is also an excellent electrical insulator and so is used in hard wearing insulating material electrical products.

• It is unusually slippery for a polymer and is used in kitchen equipment e.g. to make non-stick coatings for pans - food adhesion is reduced by the particular molecular properties of PTFE.

  • PTFE's low coefficient of resistance enables it to be used for moving engineering parts in machines and even humble zips etc.

• It is used in the clothing industry to make clothes fabrics that dirt finds difficult to stick to.

• Because PTFE is strong and chemically unreactive it can be used to make pipes to carry potentially corroding fluid and containers for storing and transporting chemicals.

• Its properties also make it useful as a component in the production of Gore-Tex.

• PTFE (Teflon) is hydrophobic, meaning it easily repels and shreds water AND it is also oleophobic and shreds oil in the same way it repels water. Both types of these liquids has difficulty in adhering to a PTFE surface.

• Because surfaces coated with Teflon are both oleophobic and hydrophobic, they are not readily wetted by either liquids allow them to be used in a variety of applications where a clean surface is particularly desirable e.g.

  • Cleaning surfaces is easier and more thorough AND a surface can be made self-cleaning.

  • PTFE coating materials bond with many different surfaces at the nano-scale to create easy-clean, water and oil resistant, anti-smudge coatings.

  • This type of coating is being used for eyeglasses and other surfaces such as flat panel displays, and stainless steel.

• For more on specialised PTFE products see Part 6d. below.

Part 6d. Hydrophobic coatings and super hydrophobic coatings

• As already mentioned above, specialised PTFE coatings create a water-repellent (hydrophobic) and oil-repellent (oleophobic) surface to repel water, dust, oil, and dirt on surfaces allowing for easy-clean glass and easy-clean metal.

• There are many other materials such as silicone polymers that give materials good water repellent (hydrophobic) properties but water repellence varies from material to material - not all are super repellent!

• Ultrahydrophobic (or superhydrophobic) surfaces are highly hydrophobic, i.e., extremely difficult to wet. A droplet of water impacting on these kind of surfaces can fully rebound like an elastic ball.
• How do superhydrophobic surfaces work? e.g. coating a surface with a super hydrophobic spray or dipping the object into a super hydrophobic liquid.
• A superhydrophobic coating is a nanoscopic surface layer that has extremely effective water repellent properties.
• The intermolecular bonding forces between water and the nanoscale super -hydrophobic coating are extremely weak, hence the water forms into droplets and cannot wet the surface. This explanation applies to all hydrophobic water repellent surfaces, but superhydrophobic materials have an outstanding water repellence.
• Superhydrophobic coatings rely on a delicate micro or nano structure for their repellent properties.
• Unfortunately, this delicate structure is easily damaged by abrasion or cleaning so these coatings are usually applied to surfaces not prone to wear e.g. electronic components.
• Super hydrophobic coatings reduce friction in fluids e.g. a boat hull moving through water, but here, the constant friction would require constant re-application of such a coating to maintain a high degree of performance.
• Apparently silica-based super hydrophobic nanoscale coatings are the most cost effective to use?
• Super hydrophobic materials can also be made from e.g. titanium dioxide (TiO₂) nanotubes, carbon nanotubes, metal oxide-polystyrene formulations.
• I assume that many of the applications of PTFE described above, can also be done with the newer superhydrophobic coatings, and even better i.e. from hydrophobic to superhydrophobic?
  • Note that most applications mentioned are to do with an exterior surface coating, but Gore-Tex is a multilayered fabric material, but I assume its hydrophobic nature is not as good as a super-hydrophobic?
• Nature note: As ever, 'mother nature' through evolution, was there first! There are many super hydrophobic surfaces in nature, including lotus leaves, butterfly wings and duck feathers etc. where a water repellent surface is desirable!