Posted Sep 28, 2015 12:00:00 AM
By Don Green
Why Knowing the Difference between PTFE Coatings Matters
First things first: Do you know that there are two formulation options for PTFE? Don’t feel bad if you don’t—most medical device customers don’t. Each formulation—aqueous-based and solvent-based—has its place and application. What you need to know is that it’s important to understand how to select the appropriate formulation.
The answer lies in the application “consequences.” Choosing the wrong formulation may create performance issues with the coating in a particular application. For example, a mandrel that has been coated with a solvent-based PTFE will not provide the best release performance (non-stick) compared to an aqueous-based system. In difficult and tortuous path medical applications, a solvent coating may be best given the strong adhesive functional requirements.
The Chemistry Connection
Let’s take a quick look at the chemistry behind this. All PTFE (polytetrafluoroethylene) is made with a free radical polymerization process combining carbon and fluorine atoms. The resulting polymers are high molecular weight, long straight chain molecules. These polymers exhibit excellent low friction properties, as well as release, heat resistance, and chemical resistance characteristics.
In aqueous-based PC 20-405 and DuPont’s 850G-314, the PTFE active ingredient goes directly from the original polymerization process into the final coating dispersion. Then chromic acid, pigments, and surfactants (specifically, replacements for perfluorooctanoic acid, PFOA), are added, which help promote substrate adhesion during the high-temperature cure cycles between 700oF and 750oF.
On the other hand, solvent-based PTFE coatings, such as the PC 4006 series and the PC 403 series, use a dry, fully-cured, ground powder PTFE. This PTFE micro powder is added to an organic solvent containing a binder and pigments (no chromic acid or surfactants). Since the PTFE is already fully cured, only the final cure of the binder has to be reached to promote maximum adhesion. This is typically hundreds of degrees lower than the aqueous dispersions, and it helps decrease oxidation of exposed metals. The lower cure temperature for solvent-based coatings is a perfect fit for nitinol applications.
Exhibit 1 presents a series of FTIR spectra showing the differences between aqueous-based PTFE (PC 20-405) and the solvent-based coatings (PC 8-403BL, PC 4006G and PC 8-403G). With the exception of the various pigments in these two coatings, the differences between the two chemistries is the addition of the binder in the solvent system. In all cases, the PTFE stretch appears in the same 1100 to 1200cm-1 region.
Exhibit 1: FTIR of Aqueous-Based and Solvent-Based PTFE Coatings
- The surface finish of aqueous-based coatings is very smooth regardless of the deposition method
- Deposition thicknesses are extremely thin given the particle size of the dispersion that is measured is in the sub-micron range
- Excellent low friction and release
- Target applications—mandrels and tight tolerance guide wires and core wires
- The surface finish of solvent-based coatings is less smooth, and under high magnification resembles the surface of an orange
- Deposition thicknesses are not as thin as aqueous-based coatings given that the particle size of the PTFE micro powder in the solvent chemistry is measured in the 4 to 7 micron range
- Good low friction and release
- Target applications—guide wires and core wires and low-cure metals, like nitinol
Precision Coating provides two types of medical-grade PTFE coating chemistries for medical devices: aqueous-based PTFE dispersions and solvent-based coatings. We’d be happy to talk with you about which of these two coatings is the right choice for your next job. Contact us so we can evaluate how we can provide you with the best solution.