Powder Coating Chemistry Options for Medical Applications

By Kevin Biller

The performance of coatings for medical instruments and devices encompasses a very unique technology space. Not only do they have to protect and beautify the item, they also need to withstand a myriad of biological and sanitizing protocols.

The most common requirements for medical powder coating are found in the international standards: ISO 13485 Medical devices — Quality management systems — Requirements for regulatory purposes and ISO 10993-1 Biological evaluation of medical devices - Part 1: Evaluation and testing within a risk management process.

ISO 13485
This is the gold standard quality management system for materials and processes pertaining to medical devices ranging from surgical instruments and hardware to implantable appliances and devices.

“[It] specifies requirements for a quality management system where an organization needs to demonstrate its ability to provide medical devices and related services that consistently meet customer and applicable regulatory requirements. Such organizations span the life cycle of a product, including design and development, production, storage and distribution, installation, or servicing of a medical device and design and development or provision of associated activities (e.g., technical support). ISO 13485 can also be used by suppliers or external parties that provide products, including quality management system-related services to such organizations.” (source: www.iso.org/iso-13485-medical-devices.html)

ISO 10993-1
This standard governs the biological evaluation of medical devices within a risk management process and covers the following issues:

• “the general categorization of medical devices based on the nature and duration of their contact with the body;
• the evaluation of existing relevant data from all sources; the basis of a risk analysis;
• the identification of additional data sets necessary to analyze the biological safety of the medical device and,
• the assessment of the biological safety of the medical device.

In addition, this standard applies to evaluation of materials and medical devices that are expected to have direct or indirect contact with the patient’s body during intended use and the user’s body or if the medical device is intended for protection (e.g., surgical gloves, masks and others). ISO 10993-1 is also applicable to biological evaluation of all types of medical devices including active, non-active, implantable and non-implantable medical devices.

This document also gives guidelines for the assessment of biological hazards arising from risks, such as changes to the medical device over time, as a part of the overall biological safety assessment; and breakage of a medical device or medical device component which exposes body tissue to new or novel materials. Other parts of ISO 10993-1 cover specific aspects of biological assessments, related tests and device-specific standards that address mechanical testing.” (source: www.iso.org/standard/68936.html)

Powder Coatings for Medical Applications
Both thermosetting and thermoplastic type powder coatings contain chemistries suitable for medical applications (Table 1). Besides requiring excellent adhesion to often difficult to coat substrates (e.g., stainless steel, anodized aluminum, brass, etc.) and film properties such as abrasion and scratch resistance, coatings for medical devices need to resist sterilization protocols such as exposure to strong detergents followed by autoclave. The autoclaves in hospitals and clinics use steam and pressure to kill microorganisms such as bacteria and spores. Coatings designed for surgical equipment need to withstand multiple autoclave cycles throughout the expected life of the instrument.

Thermoset powders chemically react in the curing oven and form an extensive chemical network that no longer flows with subsequent heat. Thermoplastic powders are based on high molecular weight polymers that melt and flow in the curing oven. Subsequent heating cycles, if high enough in temperature, can reflow a thermoplastic coating.

A variety of metal substrates can be coated with these coating chemistries including stainless steel, titanium alloys, high temperature aluminum alloys, anodized aluminum, brass, and copper. The substrate must be able to withstand the film formation and curing process temperatures. Metal preparation requires thorough removal of any dirt and oils.

In addition, media blasting is commonly used to create a surface profile to enhance coating adhesion. Diecast metals require a degassing process, essentially a “prebake,” prior to the application of the powder to drive out any trapped air and/or moisture in the substrate’s porosity.

Medical grade thermosetting epoxy powders are applied by electrostatic spray and can be used at relatively thin films as low as 1.5 to 2.0 mils (38 to 50 µm). Thermoplastic types are typically applied in thick film via fluidized bed application, although some special grades can be electrostatically sprayed. Fluidized bed coating requires the parts to be preheated to fairly high temperatures of 450 to 700 degrees Fahrenheit (230 to 370 degrees Celsius). The heated parts are then dipped into a bubbling bath of suspended powder. The coating immediately melts and adheres to
the hot part. A post heating process is typically used to smooth out the coating. The preheating/dipping process can be repeated to build thicker films. Films as thick as 50 mils (1.27 mm) can be applied, however coatings of 10 to 20 mils (250 to 500 µm) are more common.

Powder Coatings for Hospital Carts
Coating engineered boards for nurses’ carts are a unique medical application for powder coatings. These are often constructed of MDF (medium density fiberboard) and require a finish that is durable, scratch resistant and chemical resistant. Both UV curable and thermosetting powder coatings are used to meet these requirements.

UV curable powder is applied electrostatically to preheated boards to deposit an even film thickness that conforms well to board faces and machined edges. The coated boards are then heated with infrared energy to allow the powder to melt, fuse, and form a continuous film. Soon thereafter the still warm coating is exposed to high intensity UV light which causes the coating to rapidly crosslink and cure. The entire process can take as little as four to five minutes.

Novel thermosetting powder coatings have been recently developed for MDF and other heat sensitive substrates. These innovative coatings are applied similarly to UV powders, usually with a preheat stage followed by electrostatic spray. The powder is then melted, fused, and cured using infrared energy at peak temperatures around 265 degrees Fahrenheit (129 degrees Celsius) for around five minutes.

Both UV curable and thermosetting powders designed for MDF are available with antimicrobial performance. The antimicrobial performance is to protect the coated item and not the health of the general public.

Servicing the Medical Equipment Industry
To supply the medical device industry, it is essential to thoroughly understand the coating requirements and substrates, as well as the stringent regulations for medical applications. Of utmost importance is delivering a coated product with long-term performance vis-à-vis the intense rigors of a hospital or clinic environment.
This usually entails multiple uses with a demanding sanitizing process between each use. In addition, the coating must be inert to human tissue and typically possess a hard, durable, heat resistant surface with excellent slip properties.

A thorough understanding of regulations, both local and international, is requisite. Most important are the above-mentioned ISO 13485 and ISO 10993-1 standards. Operating in the medical coating arena is not for the faint of heart.

Kevin Biller is technical editor of Powder Coated Tough and president of The Powder Coating Research Group.