PTFE and Silicone Performance Coatings

PTFE Molecule

PTFE

Polytetrafluoroethylene (PTFE) is a fluorocarbon plastic -- a plastic in which the hydrogen normally found in association with carbon in organic materials has been replaced by fluorine.

This produces a number of remarkable properties one of which being that PTFE is reputedly the most inert substance known. The outstanding properties of PTFE are:

  • Inertness: affected only by a number of uncommon substances at high temperature
  • Low friction: co-efficient 0.04 - 0.10, depending on load and surface speed
  • Outstanding electrical insulation and di-electric properties
  • Extreme heat and cold resistance: -190°C to + 260°C
  • Clean release from sticky materials ("non-stick")
  • Chemical, corrosion, and moisture resistance
  • Easy cleaning (nothing bonds permanently)
  • Resistant to mildews and fungi
  • Food contact approval

PTFE is best known generally as a non-stick lining in cooking utensils and on cutting tools, and as a thread seal tape. There are a vast number of industrial uses for which PTFE is available as block, sheet, rod and tube, all of which can be machined in similar fashion to metal. PTFE film can be formed by casting, extrusion or skiving (veneering) processes.

There are a number of other fluorocarbons which have some degree of similarity to PTFE. The closest in performance properties, are commonly used for processing and coating:

Fluorinated Ethylene Propylene (FEP). Working temperature is -260°C to +205°C. As it forms a proper film surface (as opposed to the micro-porous sintered surface of PTFE) it has lower permeability to most liquids and gases (and resists the locking on of ice). Impact strength and wear resistance are also higher.

Perfluorinated Alkoxy (PFA), a co-polymer of PTFE. Working temperature is -260°C to +260°C. It has similar properties to FEP but with the higher working temperature of PTFE.

PTFE COATED PROCESS FABRICS

Coating

PTFE Group of Companies uses PTFE in the form of water-based (aqueous) dispersion to coat flexible woven, and some non-woven, materials such as glass fibre cloth (usually referred to as glass cloth, or simply "glass"), aramid fabrics and woven wire. These high performance substrates lend the polymer additional dimensional stability and mechanical strength. Only materials which stand the very high processing and application temperatures can be coated. A variety of techniques can be used in processing to improve the tear strength and resistance to creasing of the finished fabric, provide conductivity (anti-static properties), or resistance to oil and fats.

Colour

The natural colour of coated fabric is light to medium tan, sometimes darker. This is normally not controllable, as the colour is produced by the caramelisation of the weaving sizes and lubricants in the various cloths when heated during PTFE coating. Pigmentation is possible. PTFE Coated Glass Fabric (PTFE-Glass) In the bulk of applications glass cloth provides the most cost effective substrate, and more than matches the PTFE's upper recommended operating temperature of 260°C and lower static operating temperature of -190°C. In dynamic conditions the coated fabric can be used down to -50°C. The E-glass (electrical grade) glass used has a high strength-to-weight ratio and excellent electrical properties.

A wide range of cloth types are coated, and a combination of glass cloth and weight of coating is available to suit most applications. PTFE coated glass fabric is defined by: Base glass cloth style (coaters worldwide use similar types) (the US style reference as used in this paper is the most common found); Weight of PTFE coating; Pre-treatments to base cloth or polymer; Whether one or two-side coated; Whether calendered (depending on processing method).

Thickness /Gauge

PTFE-glass fabrics are frequently referred to by thickness, in inches or metric measure. The most popular are: Original inch size 0.003", 0.005", 0.006", 0.010". In the USA these are known as 3mil, 5mil, 6mil, 10mil and ‘3thou, 5thou, 6thou, 10thou’ in the UK. In metric units, they are 0.076, 0.127, 0.15, 0.25 (mm). US glass style numbers used are 108, 116 ,116 and 128 respectively.

Intermediate thickness' are possible, and much heavier fabrics are used in process industries and in architectural applications. As most coaters aim to produce a fabric of given weight rather than thickness, the most accurate comparison is by base glass cloth type and coated weight rather than thickness, although thickness can be important in some applications.

Some caution is necessary when making comparisons: fabrics varying in weight between 200g/m2 and 270g/m2 can casually be called 5thou; however, as most of the weight over 200g is on the fabric surface - where it is most useful in most applications - it is clear that weight gives a better indication of quality and cost than thickness, so long as the base cloth is comparable.

Silicone

Silicone rubber was developed by the American General Electric company. The largest producer is now Dow Corning who was originally licensed by General Electric. Other manufacturers of the base polymer include Bayer, ICI, Rhone-Poulenc and Wacker.

The material is available in many formats, including heat-cure rubber (similar in processing to other types of rubber), and RTV (Room Temperature Vulcanising) forms. The latter are not truly vulcanised (that is, with sulphur), but hardened from a viscous liquid state by special catalyst systems.

The outstanding properties of silicone rubber are:

  • Extreme heat and cold resistance (-115°C to 260°C)
  • High release from sticky materials (Non-stick)
  • Easy cleaning
  • Resistance to many common chemicals
  • Mildew and fungus resistance
  • Outstanding electrical insulation
  • FDA clearance for food processing and handling (some grades)

Silicone rubber is best known generally as a one-part RTV sealant (particularly in bathrooms), and as tubing on the bags in which increasing numbers of liquids are packed. Another well-known use is as the two part RTV adhesive for the protective tiles on the space shuttle. Silicones are also used in many other forms, particularly as oils and greases. The adhesive coating on Biscor's high temperature adhesive tapes is a silicone. 

Silicone Rubber Coated fabrics

PTFE Group uses the elastomer as dispersion and in other forms to coat glass cloth. Silicone rubber coated fabrics are not coated to the same generally accepted specifications as PTFE coated materials, and an almost infinite range is possible. PTFE Group produces fabrics from 0,17mm to ca. 1.00mm thick.