Pre-Treatment Process Solves Adhesion Issue

Prior to 2006, commercially available aqueous-based polytetrafluoroethylene (PTFE) coatings utilized perfluorooctanoic acid (PFOA) as a principal surfactant. PFOA was a highly efficient additive and allowed PTFE applicators to coat a wide variety of substrates.

In 2006, PTFE formulators were required by the U.S. Environmental Protection Agency (EPA) to eliminate PFOA from all aqueous-based PTFE formulations by 2015. Companies began to create new formulations that were initially touted as drop-in replacements. (See Figure 1)

The performance of some of the reformulated coatings in medical wire applications were found to be generally equivalent on a range of application tests and were adopted. With time, however, customers began observing intermittent delamination when coated product was exposed to saline solution or high-alkaline cleaners.

Failure Mode Investigation

In order to form an effective PTFE coating with strong adhesion, preparation of the substrate is critical. In the case where the substrate is contaminated or non-uniform, a strong surfactant can compensate for less-than-adequate surface preparation. PFOA was just such a surfactant. It was quite effective at enhancing the spreadability and wettability of the coating, even over less than pristine surfaces, to achieve a reliable bond. The replacement surfactants, unfortunately, are less effective in this capacity and are more sensitive to surface conditions. (See Table 1)

Investigation into the issue of delamination revealed this to be the case. The root causes were related to the cleanliness and the general condition of the wire surface. It was clear that these conditions varied widely depending on the wire source. After all, each supplier has its own set of parameters and materials of contact for forming, grinding, and preparation methods. These processes introduce contaminants and also serve to disrupt the surface structure of the wires thus treated. Further, the wire subassemblies will exhibit downstream hysteresis of these upstream events.

Further complicating the situation was the fact that there was no uniform test regimen used to characterize coating adhesion. As well, the test procedures available at the time were not able to reliably detect the deficiency.

A range of inputs was evaluated to identify specific failure mechanisms. It is well documented that stainless steel in its normal state spontaneously forms a thin layer (i.e., molecules thick) of primarily chromium oxide, which contributes to a number of its performance characteristics. This oxide layer is responsible for interacting with the acids that are present in water-borne PTFE dispersions, and promotes bonding of these types of coatings securely to the surface. The oxide layer is critical to coating adhesion. It was discovered that the aforementioned wire forming steps disrupted this oxide layer and that cleaning steps to remove associated oils or greases were not as effective as they needed to be.

Exposure of the coating to saline solution (to which the products are exposed in the field), was found to be a trigger for the failure, if adequate adhesion had not been achieved.

The Solution

Since the replacement surfactants are much less effective than PFOA, it was determined that current surface preparation methods needed to be revisited. Studies demonstrated that it is necessary to re-establish the disrupted metal oxide layer to provide a uniform surface allowing complete wetting and spreading of the coating. The result is a securely bonded coating.

The “fix” is in preparing the surface to accept the coating. In simplest terms, the surface preparation improves uniformity, and by extension, wettability of the substrate. Anomalies and surface conditions that cause weak adhesion are removed, thus eliminating the causes of intermittent delamination. The resultant technology:

• Establishes the right surface conditions to promote formation of a strong bond between substrate and coating;
• Does not physically alter the surface of the wire and does not introduce any new or additional chemicals to the current process; and
• Delivers robust and complete adhesion of the coating to the wire.

Also, part of the solution was the development of an appropriate test that reliably identifies the presence of the defect. A simple and effective test that has been used historically to assess coating adhesion is ASTM D3359. This test, although perhaps effective for evaluating other types of coating, proved ineffective in detecting the occurrence of delamination.

It wasn’t until the test was coupled with a pre-conditioning step in which samples are exposed to a conventional 0.90% (w/v) solution of sodium chloride in water prior to testing that the defect could be consistently and reliably detected. Pre-soak times of 20 minutes are adequate, although some opt for a longer soak time (up to three hours) to try to model exposure time indicative of actual procedures. Regardless, the saline soak & tape test (SSTT) is now the most commonly used test method to determine adequate adhesion of PTFE coatings to stainless steel wires.

It should be noted that an ISO test (ISO 11070:1998(E) – Annex F) is also used to determine viability of the coating in these applications. However, similar to the tape test, the defect can’t be consistently and reliably detected until the test is coupled with the saline soak preconditioning step.

Qualification of the Pre-Treatment Process

With the qualification of this solution (i.e., the pre-treatment process), which introduces no other change to the existing application process, delamination has been eliminated and prevented from recurring.

The qualification establishes documented evidence that this pre-treatment process consistently produces products that meet pre-approved specifications, including adhesion to the substrate and surface friction. These efforts have led to the development of a definitive and validated test method that ensures resolute adhesion of the coating to the substrate, providing consistent high-bonding strength for PTFE coatings on stainless steel medical devices. (See Figure 2)

Qualification Objective and Scope

The qualification was designed to test the critical attributes that could be affected by the inclusion of the pre-treatment step and mitigate any regulatory implications that would be of concern regarding the effectiveness of the pre-treatment, the effects on final properties, the effect on the chemical structure of the coating, and the presence of any residual components.

Qualification testing included adhesion (SSTT), coating appearance, coefficient of friction, coating thickness, OD measurement, bare and coated wire diameters, tensile and elongation strength, high-magnification visual of substrate surface condition & chemistry (SEM), cured coating chemistry (FTIR Analysis), and friction/durability testing.

Qualification Samples

Process

• The process was qualified on 304 stainless steel wires coated with the zero-PFOA containing PTFE coating in a diameter range of 0.007" to 0.032"
• Five sample groups of 1,000 units each at the following diameters were evaluated in the qualification: 0.007", 0.010", 0.013", 0.0215", and 0.031"

Qualification Results: Adhesion (Saline Soak & Tape Test)

The most important attribute tested was the effectiveness of the pre-treatment step with regard to adhesion of the coating to the substrate. Results, shown in Table 2, indicate that the pre-treatment step is effective at preventing delamination.

The following attributes were also tested:

• Coating appearance
• Coefficient of friction
• Coating thickness
• Bare wire diameter
• Wire tensile strength and elongation
• High-magnification visual and surface chemistry
• SEM analysis
• Final chemical composition of the coating

All these analyses yielded the same result: proper pre-treatment provides PTFE-coated wire equivalent in all ways to untreated wire, except that the adhesion is uncompromised and reliable.

Final Analysis

The proper pre-treatment process eliminates the factors leading to delamination of the zero-PFOA containing PTFE aqueous coating from stainless steel wire substrates. The qualification was conducted in accordance with industry-standard quality systems and has been released to market.

Results of the qualification support these conclusions:

• Proper pre-treatment eliminates delamination of aqueous PTFE coating.
• This pre-treatment step does not pit, degrade, or in any way remove material from the wire surface.
• This pre-treatment step does not compromise the mechanical properties of the wire.
• Proper pre-treatment does not result in the loss of frictional performance.
• This pre-treatment step does not change the surface chemistry of the wire, nor does it leave any residual chemical behind.
• Proper pre-treatment does not change the final chemical composition of the applied coating.

Neurovascular OEM Qualification and FDA Master File

A proprietary new technology was a critical component in the qualification of a PTFE-coated neurovascular guide wire. A global medical device manufacturer of neurovascular wires became the first entity to qualify the process that eliminates delamination. This effort led to the development of a definitive and validated testing protocol to ensure tenacious adhesion of the coating to the substrate. The Saline Soak & Tape Test was employed and is now an industry standard.

Introducing the new technology involved a collaborative effort between the coating service provider that created the process and its downstream partners (both CMO and OEM). Ultimately, an FDA Master File was completed supporting the viability of the new surface preparation step.

Questions to Consider

Q: Does the process result in loss of dimensions of the substrate?

A: No. Dimensional analysis using a dual-axis laser micrometer demonstrates there is no dimensional change.

Q: Does the process change the surface chemistry of the substrate or leave behind any new chemicals?

A: No. SEM with chemical analysis testing demonstrates that the proper process does not change the surface chemistry of the substrate and leaves no new chemicals behind.

Q: Does the process compromise the mechanical properties of a guidewire?

A: No. Tensile testing and the results demonstrate that the tensile properties of the wire have not been compromised.

Q: Does the process effect the final chemical composition of the coating on the substrate?

A: No. FTIR spectrophotometric analysis and the results indicate that the chemical composition of the applied coating is identical between wires treated with the proper process and untreated wires.

Q: Does the process compromise the frictional properties of the applied PTFE coating?

A: No. Frictional testing demonstrates that frictional properties are comparable between treated and untreated wires. In fact, a slight increase in lubricity on wires treated with the proper pre-treatment process was observed.

Q: Does the process compromise the cosmetic appearance of the wire?

A: No. In fact the cosmetic appearance of wires treated with the proper pre-treatment process seems to be slightly enhanced.

This article was written by Rick LaPorte, Chief Technology Officer, Katahdin Industries, owner of Precision Coating and Medi-Solve Coatings, Boston, MA. For more information, Click Here .

---