Edited by David S. Hotter
Technical Service - Teflon
Though its claim to fame is keeping food from sticking to cookware, Teflon polytetrafluoroethylene (PTFE) resin and other PTFEs are also engineering materials. Engineers overcome numerous design changes by using the same low coefficient of friction that helps eggs slide out of the pan and onto the plate, along with the chemical resistance and service temperatures ranging from cryogenic to 500°F, to overcome a variety of design challenges. Fluoropolymers are vital to the functioning of applications such as gaskets, seal rings, bearings, pipe linings, and electrical and thermal insulators, to name a few.
As always, price is critical in material selection. And in the case of high-tech resins such as fluoropolymers, there is a premium on performance. Using reprocessed resin is one way engineers can save money. But don’t send your purchasing department out to the resin recyclers yet. The physical properties and performance of components made of virgin and reprocessed resins are significantly different.
Whether virgin PTFE resin is needed or reprocessed (repro) will do the job depends on the property levels required by a particular application. For most molded parts, the higher properties achieved with virgin resins make it the only choice. However, for some applications, lower properties are sufficient and using repro resin will cut costs.
A review of the processing steps used in molding and the differences in properties between virgin and repro PTFE resin will make the choice easier.
What’s the difference
Virgin PTFE granular resin can be processed by various types of compression molding and ram extrusion. Manufacturers use compression techniques to mold resin granules into shapes, called preforms. These shapes may be finished components or stock shapes such as sheets or billets. The preforms are then heated or sintered, by the same technique used for powdered metals, to melt granules together and reduce porosity.
Molded preforms may be within final tolerance limits. If not, secondary operations such as machining yield tighter tolerances.
Ram extrusion is used to form rod and tubing shapes from PTFE. An oscillating ram forces resin through a heated cylinder and a die. Resin particles sinter under pressure in the cylinder, which makes the process flexible enough to handle both virgin and reprocessed resin.
Before molding reprocessed PTFE into parts, contaminants — which can include lubricants, metal particles, and chemicals — must be removed. To do this, workers shred scrap and heat it to above its melting point for several hours to burn off organic contaminants. Then it is ground and treated with acid at high temperatures to dissolve inorganics. Finally, the acid is washed off, the resin is reheated to vaporize volatiles, and the cleansed resin is reground.
Repro resin particles are hard because they typically have been melted three times, and don’t adhere well enough to make preforms. Therefore, repro resin must be sintered under pressure in a mold or ram extruded.
In addition, each time it’s melted the resin’s molecular weight decreases due to molecular chains breaking down into smaller segments. The lower molecular weight also leads to higher levels of crystallinity. Because the molecules are smaller, it is easier for them to form crystalline segments while cooling. Repro has crystallinity levels greater than 80%, compared to 50 to 70% for virgin material.
All of this limits repro resin’s final mechanical properties. Those properties will also vary more widely because scrap can come from so many different sources.
Comparing the differences
A comparison of mechanical test data taken from sheets and rod made of virgin and repro resin illustrates the difference in performance expected from molded parts. Except where indicated, virgin sheet was made with pelletized resin, which results in lower porosity than fine-cut virgin granular PTFE.
Tensile properties: At room temperature, sheets made from virgin PTFE elongate more than 200% and have a break strength of
3,000 psi, compared to 100% elongation and less than 2,000-psi break strength for repro. For rod stock, the difference in tensile properties is even more dramatic. Rods made from repro elongate only 10% as much as virgin resin, and tensile strength ranges from 60 to 75% that of virgin PTFE.
Many applications using PTFE are for components performing at high temperatures. At 212°F, sheet made from virgin resin elongates twice as much than at room temperature. Elongation remains the same or decreases, however, for a similar temperature rise for sheets made of reprocessed polymer. As for tensile strength, both resins lose strength at about the same rate as temperatures rise, with virgin resin always remaining on top.
Deflection temperature: Standard heat deflection tests under load are performed with both low and high loads. For small loads (66 psi), repro has a slightly higher deflection temperature than virgin resin. However, at higher loads (264 psi), such as those more typical of actual service conditions, the difference between the resins disappears.
Impact strength and flexural modulus: Test specimens made from repro material are brittle compared with virgin resin. Notched Izod impact strength is lower for repro resin, particularly at -40°F and below. Flex modulus data for the two resins follow much the same pattern. Repro parts have flex modulus values greater than 150,000 psi, while values for virgin PTFE range from 40,000 to 90,000 psi at room temperature, depending on the resin’s molecular weight.
Creep: Deformation under load is a critical property for applications such as gaskets. In general, repro material creeps more than virgin resin. For example, repro creeps 25% more than virgin at 2,000 psi and room temperature. The difference in creep levels increases at higher loads.
Void content: Porosity, or void content, is measured using Fourier Transform Infrared spectroscopy. Repro sheet typically has a void content of about 2%, while virgin specimens have less than 1%. The difference is because repro resin particles are harder than virgin resin and are therefore more difficult to compress together under processing pressures.
Permeability: As expected, the results for permeation values are similar to that of porosity because voids create a path.
Dielectric strength: Repro sheet has a dielectric strength half that of virgin material. Factors that contribute to a lower value include porosity, contaminants, and low molecular weight. n
How to identify repro resin
There are several ways to tell whether the granules of PTFE resin or even molded parts are made from reprocessed material. Unprocessed resins are differentiated by their higher melting points. And engineers can test the tensile elongation, specific gravity, and crystallinity of finished parts.
Reprocessed PTFE resins melt at 621°F, while virgin material melts at 648°F. Melting temperatures are measured using differential scanning calorimetry. The same test doesn’t work for molded parts because the resin has already been melted once.
For parts made from reprocessed resin, specific gravity is usually higher than the 2.14 to 2.18 range of virgin resin. The significant differences between elongation and crystallinity values also make it easier to recognize repro resin.
In some cases, engineers can distinguish parts made with reprocessed resin by appearance. Repro parts may be off-white because they were sintered in a mold. In addition, scanning electron micrographs of repro parts often show voids as white spots.