The Manufacture and Properties of PTFE Micropowders
PTFE micropowders are used to reduce friction between contacting surfaces. These materials are produced in low micron or sub-micron particle size and in a range of low molecular weight. The raw materials and processes used to make the micropowders determine the properties such as surface energy, surface area, hardness and more. To understand the different properties, it may help to understand how these PTFE products are made.
PTFE is produced by the polymerization of TFE in pure water. This yields a product known as suspension grade or granular. That product is processed by compression and sintering to make parts directly or to make billets suitable for machining.
PTFE is also polymerized in water with a wetting agent. This may be used directly to impregnate fabrics or coagulated to make emulsion grade PTFE, also known as fine powder, coagulated dispersion or paste grade. Emulsion grade PTFE is used in a process called paste extrusion. The powder is wet with volatile oil to make a paste that is then extruded to produce a tape, tube, or wire cover. If the product is to be tubing or wire covering, the PTFE is then sintered. Thread seal tape is left alone, but expanded material such as Gore-Tex is biaxially stretched.
Most PTFE micropowder is made by degrading one of the grades of Natural Prime PTFE, one of the products made, or by-product from the processing of PTFE. These are many grades of PTFE and possible degrees of degradation. There may be a dozen grades of emulsion PTFE. Dispersion grade may be sintered, or unsintered. Pigments and fillers are used in some cases. Strained material behaves much differently than unstrained, as do materials in the various polymorphic states.
Although there are some special grades of PTFE micropowder made by direct polymerization, most are made by degradation using either an electron beam, cobalt irradiation, or heat. Any of the materials described may be used. Degrees of degradation may be controlled to control molecular weight and molecular weight distribution. A grinding process is used to control particle size and particle size distribution. All this will relate to the material being processed. Other solids, liquids, or gasses may be included in the processing. PTFE is an amazing material.