Tubing and Parylene Coating

Tubing is designed to contain or transport substances, typically liquids or gases, and is often constructed of plastic, metal and sometimes glass. When many people think of tubing, they envision its use in construction or mechanics, which is often specified by its material of construction as well as its dimensions, specifically OD (outer diameter) and ID (inner diameter). Wall thickness is calculated as ½ OD minus ID.

Used as much for structural purposes as conveyance, these tubes can have cross sections that are round, or square/rectangular in shape. Tubing is used for many markets including automotive, energy, engineering and precision/pressure uses. Depending on its purpose, larger-scale tubing may benefit from conformal coating, although this is less likely. For example, piping can employ liquid films of epoxy or silicone as coatings.

Conformal coating is more frequently applied for smaller, specialized tubing, used in biomedical, computer or electronic devices. In these cases, tubing is also used as a means of material conveyance, such as carrying medicines through implanted devices within the body or liquid cooling for computers.

The proliferation of microelectromechanical systems (MEMS) and nanotechnology increases the need for reliable conformal coating of specialized tubing.

Common liquid conformal coatings – resins of acrylic (AR), epoxy (ER), silicone (SR) and urethane (UR) – are impractical for both tubing uses and MEMS/nano applications:

• Their effective coating layers are far too thick MEMS and nanotechnology devices. (AR/ER/UR: 0.025–0.127 mm and SR: .051–0.203 mm)
• Liquid deposition methods – brushing, immersion (dipping), spraying – cannot be adapted for adequate coverage.

As a result, liquid applications are generally not reliable for coating tubing of any size. Applied as a wet material, brushing is slow and will not evenly coat inner tubing surfaces. Both dipping and spraying may provide a more even OD coat and better internal coverage but cannot be relied upon to provide a uniform and effective protective layer for ID surfaces.

In contrast, Parylene (XY) conformal coating can be successfully applied to tubing regardless of size. Its specialized vapor deposition polymerization (VDP) method creates pinhole-free conformal films of exceptional uniformity. Granular XY dimer material is transformed into a gas under a vacuum, during the VDP process. Its vaporous condition allows XY to travel to the coating chamber and coat anywhere on the substrate the gaseous XY reaches, including the tube’s inside surfaces.

Unlike liquid coatings, whose wet application methods prevent film materials from reaching a substrate’s obscure or hidden surfaces, Parylene covers the entire assembly. Vaporously penetrating a substrate’s surface, XY provides a protective layer both within the object’s surface, as well as to its interior, offering a further stratum of conformal protection. This applies to tubing, regardless of size or shape. Gaseous migration of Parylene into the interior of the tube allows for coating of the tubing ID. Moreover, Parylene’s ultra-thin films, typically from 0.013–0.051 mm, are manageable at levels less than one micron (1 μm/1,000 nms), benefitting MEMS/nanotechnology tubing uses. XY’s extremely thin conformal films add minimal build-up to either tubing’s OD or ID.

Truly conformal Parylene generates an easily cleaned surface of exceptional dry film lubricity, permanently bonding to film surfaces, both external and internal. It does not attract debris. XY’s ultra-thin coatings offer additional advantages for tubing:

• Adherence to the range of existing tubing materials/topographies
• Biological/chemical inertness
• Bubble-free conformability/flexibility at film thicknesses > 0.5 μm
• Low-friction lubricity
• Penetration of extremely small crevices/spaces
• Reliable dielectric/moisture barrier properties
• Resistance to bacteria/fungus growth, heat, radiation and solvents
• Strong, resilient coatings eliminate pathways for corrosive compound entry