PTFE coated fabrics

Polytetrafluoroethylene (PTFE) is a fluorocarbon plastic; that is, a plastic in which the hydrogen normally found in association with carbon in organic materials has been replaced by fluorine.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, and PTFE is reputedly the most inert substance known to man. It was discovered and developed by Dupont™, whose trade name is Teflon®. 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

High release from sticky materials ("non-stick")

Chemical, corrosion, and moisture resistance

Easy cleaning (nothing bonds permanently)

Mildew, and fungus resistance

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 (TEFLON®) COATED PROCESS FABRICS

Coating

Our suppliers have been using 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"), Nomex®, Kevlar® 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 USA known as 3mil, 5mil, 6mil, 10mil and in the UK as 3thou, 5thou, 6thou, 10thou and in Metric (mm) 0.076, 0.127, 0.15, 0.25. 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.

In recent years products of our suppliers's “BP" fabrics have become popular. The base fabrics of these are similar in weight and thickness to traditional twisted yarn types “BT”, although some tend to be thinner after processing. The yarns in "BP" fabrics have a minimum of twist, which was absolutely necessary in the past to produce a yarn which would resist the rigours of weaving in shuttle looms. They are flatter, so the fabrics need less coating of PTFE to produce a good surface finish.

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