PTFE Coated Fiberglass: Sintered versus Unsintered

The definition of “Sinter” is: “To make a powder coalesce into a solid or porous mass by heating it without liquification”.  We typically think of sintering as it relates to metal (like bronze or brass).

W.F. Lake Corp. manufactures both sintered and unsintered materials.  With sintered PTFE, we essentially have taken a PTFE powder dispersed in water and fused it using heat into a hard, slippery coating that operates at very high temperatures, is chemical resistant and very few things stick to.  These are sintered PTFE coated fiberglass products.  Unsintered products are essentially an impregnated coating of PTFE that is dried in place, but not fully fused. It has a “waxy” feel and is less abrasion resistant.

Why sintered?  Sintered PTFE coated fiberglass fabrics, tapes, belts and sewing threads are the typical form of PTFE coated textiles you see in the marketplace.  They are the work-horse products for food processing (belts and sheets), packaging (L-Sealer bars), molding (composite release sheets), flu-gas filtration (PTFE coated fiberglass sewing threads), high temperature, hot face insulation (S-2 glass, Quartz sewing threads, etc…), and many other applications.  In these cases, PTFE powder suspended in water is fused at high temperatures (sintered) to form a solid PTFE coating.

Why Unsintered?  We are unique in that we also coat many of our fiberglass yarns with unsintered PTFE.  This process requires precise temperature control to essentially “bake” the PTFE particles without fusing them together.  This leaves a waxy type coating that has both penetrated the fiber bundle and leaves a portion of the coating on the surface of the yarn. In most cases, these yarns are braided over wire conductors or made into lacing tapes and tie cords. The advantage an unsintered PTFE coating is that it allows for further processing and bonding of subsequent layers of PTFE; in the form of PTFE films, coated fiberglass fabrics or even additional PTFE dispersions.  When all of these are finally fused, or sintered, our customers form a homogeneous coating that very effectively resists chemicals, operates at high temperatures, will not burn or support flame, will not rot or support fungus and provides excellent electrical insulation.

Most of us use sintered PTFE products virtually every day.  Pots and pans, baking sheets, even breathable, water repellent rain ware (and sometimes vascular grafts!).  Unsintered PTFE products in daily use are less prevalent, but are available as Thread Seal Tape (unsintered PTFE film), packing (valve stems, shaft logs) and gaskets (valve stems), etc…  PTFE is an amazing material.  Give us a call with any questions or application opportunities… we’re here to help!

Helpful Tips for Sewing with PTFE Coated Fiberglass Threads (and other high temperature threads)

PTFE Coated Fiberglass, PTFE Coated S-2 glass, PTFE coated Quartz and other high temperature, composite sewing threads offer amazing operating characteristics. Unaffected by most chemicals, they will not burn, will not support flame, will not rot or support fungus. Our PTFE coated quartz thread operates to 2,000 deg. F. Amazing! However, you wouldn’t use these threads unless you have to, not necessarily because they’re overly expensive, but because they require a special skill set in order to make them work. If your application needs these properties, however, there are no alternatives. I like to say this is OK because it prevents everyone with a sewing machine in their garage from becoming your competitor! It’s not as easy as changing a bobbin and switching from polyester to S-2 fiberglass with Inconel wire twisted into the thread!

As mentioned, this is not a drop-in substitute for an aramid, polyester, or any other thread. There is a learning curve to go through if unfamiliar with this type of thread. The very unique and extreme operating properties of PTFE coated fiberglass (or S-2 glass, Quartz, wire inserted, etc.) threads require operator training and machine adjustments to make them work acceptably.

First of all, a factory lubricated thread is going to be much easier to use. This lubricant is applied after PTFE coating. We offer two high speed thread lubricants. X80 is a silicone/mineral oil based thread lubricant and is, by far, the most popular choice. This product is ideally suited for extreme needle heat environments when sewing through multiple layers of thermal insulation, like silica blankets, ceramic fiber mats, texturized glass, etc… We also offer a wax based thread lubricant (X50) that is used occasionally for lighter duty applications. In a few cases, customers will “post bake” the final product to remove any residual oils.

With or without the use of lubricants, the following suggestions help dramatically.

SLOW DOWN. Especially if un-lubricated thread is being used.

Change all hooks, needles, tension guides, etc. Anything that touches the thread must be free of burrs, gouges, etc. These items will need to be changed regularly.

Be sure the tension device where the thread comes out of the bobbin case is new and burr-free.

Use the correct needle size. We’re happy to help out with a suggestion.

Be sure there are no grooves where the thread passes through the hole in the walking foot. No rubbing or abrasion points.

If sewing with a wire inserted (Stainless steel, Inconel or other), try placing a nylon stocking over the thread spool and pass the thread through a hole in the top of the stocking. This helps prevent free-spooling of the thread and minimizes any “kinking” due to the steel or wire insert.

There will likely still be breaks occurring, but the properties of these threads are outstanding and we must sometimes put these difficulties in order to take advantage of these amazing products.

As always, let us know if you have any questions or comments.