Plastics and polymers were first being produced, whether on accident or on purpose, in the early 1930s. Dupont's Teflon, or PTFE, is probably the most widely known polymer because of its uses in cooking as a non-stick coating for pots and pans. While there are lots of other polymers out there, there are only a few that have as many uses as PTFE, one of which is Parylene.
Parylene was developed by a chemist named Michael Szwarc while he was running experiments on chemical bonds between carbon and benzene rings. While heating para-xylene, he discovered a precipitate in his equipment that turned out to be small and tube-like. He correctly identified these tubes as the polymerization of p-xylene. After a brief period known as Szwarcite, Parylene soon found uses in the medical field as an excellent hydrophobic barrier, but has been found to have plenty of other uses in electronics; metal, rubber, and surface protection from corrosion and outside elements; and as a friction reducing coating especially with needles.
PTFE's discovery, on the other hand, was purely accidental. While working with gasses for refrigeration in the Dupont laboratories, Dr. Roy Plunkett thought that a canister containing TFE was not working. After cutting the canister in half, he discovered a white flake that had developed in the tank and correctly guessed that the flake was a polymer. After conducting several tests on the flakes, since TFE was widely thought to be impossible to polymerize, Plunkett discovered that it was insoluble in anything he tried, as well as being completely inert. The first applications for PTFE were on the seals for the atomic bomb, but it also worked as the nosecone for proximity bombs because it is transparent on a radar and resists electricity.
Parylene was the first vapor deposited polymer ever discovered, and because of the vapor deposits and the fact that no solvent or catalyst is used to cause the polymerization it has a one hundred percent yield, which makes it an extremely efficient polymer to manufacture. Because it is hydrophobic and biostable, parylene has been used extremely effectively as a coating for medical tools, instruments, and hoses. It's strong resistance to corrosion make it an excellent metal coating for scalpels, hypodermic needles, and other metallic tools. It also works as a micro barrier since its surface is impermeable above thicknesses of 1.4 nanometers. Its uniformity helps it adhere to sharp edges and points, again pointing to its widespread use in the medical field.
Unfortunately, because of its formation, it cannot be applied through a solvent. This means that the only way to coat an object in parylene is during the production of the polymer which occurs in a vacuum. While the object to be coated remains near room temperature, which aids in the safety of the process, and the coating is universal and uniform, it does mean that the polymer cannot be put into an aerosol can or produced en mass for consumer use.
PTFE can be made in one of two ways, each resulting in a different looking product, but by and large the same end result. With suspension, TFE is polymerized in water and results in the PTFE forming grains, whereas dispersion causes the PTFE to form as a milky paste. Both the paste and grain are processed and used to coat various products. Although PTFE itself is non-toxic, some of the byproducts of the manufacture process are toxic and at high heats the PTFE itself can emit toxic gasses.
Conformal coatings are surface treatments applied to a wide range of products and devices used for aerospace, automotive, biomedical, consumer, military and numerous other purposes. Their primary objective is providing a protective film that supports a selected device’s ease of use, operating function, and service life, through an exceptional variety of working environments. Liquid Teflon (PTFE) and parylene are two of the more widely used hydrophobic conformal coatings.
Liquid PTFE and Parylene Compared
Liquid Teflon’s corrosion resistant qualities are well documented; repelling nearly everything, the bond that exists between its carbon and fluorine atoms is so strong, the substance is nearly bullet resistant. Its lubricating properties greatly reduce friction and component wear during operation, particularly for machines with sliding parts, effectively lubricating bearings, gears, slide-plates and other moving components, more reliably than most competing covering materials. Amazingly, it also plays a role in surgical interventions as a grafting material.
Teflon displays resistance to chemical reaction, corrosion, and stress-cracking. In addition, PTFE coatings:
PTFE hydrophobic coatings enhance lubricity, a quality desirable for many applications, ranging from medical implants to aerospace/automotive mechanics, where decreased friction between components improves function and prolongs operational life. PTFE also has a lower co efficient of friction than Parylene, which normally would make it the coating of choice for applications where greater levels of lubricity are essential to enhanced performance. However, in many cases application-specific considerations make parylene a better choice.
Despite their advantages, liquid Teflon cures at temperatures and according to schedules that interfere with the basic thermo-mechanical characteristics of many of the components they coat, such as those using nitinol or similar alloys. Minimizing unintended transformation of these alloys’ thermo-mechanical properties while retaining sufficient performance lubricity is a major challenge for future liquid PTFE applications.
PTFE coatings can also produce particulates during prolonged operation. When used for medical purposes, these minute particles of matter can become embedded in patients’ tissue or bloodstream, potentially endangering bodily function and health; particulates can interfere with the performance of non-medical components as well. In contrast, hydrophobic parylene seldom dissociates or separates from the substrate during use, making it a superior option in cases where particulate development is a concern. Parylene’s generally improved lubricity in these cases transforms “sticky” substrate surfaces into those that are smooth and non-resistant. These advantages in relation to PTFE are also transferred in many aerospace, automotive, consumer and military applications.
An industry standard conformal coating material, parylene is applied via a unique chemical vapor deposition (CVD) process, wherein the gaseous parylene penetrates deeply into the substrate surface, covering even the most isolated or obscured regions of its topography. This quality makes parylene very useful for adaptation to microelectricalmechanical systems (MEMS) and nanotechnology applications, as well as larger-scale product treatments. Like liquid PTFE, parylene is noted for its durable lubricity, generating long-lasting and dependable barriers to biological, chemical, electrical, and mechanical conditions that can endanger the performance of electronic components used both for specialized purposes and in everyday life.
Parylene also provides:
Both PTFE and parylene are adaptable to a wide range of product applications. Choosing between the two is always a function of the specific uses of the component and its operational context. For instance, although PTFE is a harder coating than parylene, and is often a better option for many mechanical uses, it does tend to crack and wear. Thus, parylene is a superior film choice in specific cases, such as laser welding. These considerations need to be analyzed in all cases before deciding between them.
Parylene and liquid PTFE each have their own performance benefits and disadvantages. Selecting the appropriate hydrophobic coating for biomedical, consumer or other products is essentially determined by the specific uses and operating conditions of each device. Quality control and cleanliness testing are supported by such methods and instruments as:
Comparative performance testing of the device within the context of its intended application helps render a reliable decision but, despite its somewhat higher consumer profile, liquid PTFE lacks the overall range of product adaptations available with parylene.
To this extent, parylene’s CVD process is often superior to Teflon’s liquid application methodologies for assuring reliable conformal coating. Typically applied by dipping or spraying, PTFE is prone to bridging, edge-effect, or pooling, developments that limit the true conformality of the resultant protective film. Parylene CVD generates a micro-thin, authentically conformal film. These conditions position parylene as a generally better coating solution where dimensional tolerances are physically constricted (MEMS/nano technologies), and for components with more complex topographies that require exceptionally reliable conformal treatment.