Fluorooxidation Surface Pretreatment
by Scott Sabreen
Figure 1: Left untreated, the polyethylene particles (left) do not wet or disperse in water, while the treated polyethylene particles (right) readily disperse in water.
Fluorooxidation (F/O) is a surface pretreatment for polymers and elastomers that is conducted inline at ambient conditions. While the science of using Fluorine (F2) gas for surface modification has been well understood for more than 50 years, it has not been widely accepted primarily because of the safety concerns in using Fluorine gas. The significant breakthrough in fluorooxidation as a commercially viable pretreatment for yielding high-strength adhesion bonding is its safe gas processing and eco-friendly neutralization. Small scale, computer-controlled electrolytic cells enable the Fluorine gas to be used safely at low concentrations and low pressures.
An advantage of F/O is its capability to effectively treat large and small surface areas of complex geometry (remote/isolated regions) ranging in size from capillary to large diameter tubes and housings. F/O is an oxidation process, unlike flame and electrical discharge pretreatments which are glow discharge gas phase surface oxidation plasmas. Compared to chamber pressurized RF cold gas plasma treatment, F/O is conducted at ambient conditions. During the F/O processing, a substrate surface is exposed to Fluorine gas and other reactive and non-reactive gases in a controlled chamber. Similar to gas phase surface oxidation processes, oxygen-containing functional groups, such as hydroxyl, carbonyl and carboxyl are created making the surface wettable and very polar. These chemical and physical mechanisms are necessary to achieve strong adhesive bonds.
The chemistry of the F/O process can be customized to achieve specific surface functionality. For example, if a fiber is to be treated at very high throughput rates, higher gas concentrations can enable adequate treatment with an exposure time of a few hundredths of a second. A second example is of a polymeric powder being treated. Burning and dust explosions must be avoided, so a low concentration of the reactive gas is used.
F/O is used to surface modify ultra-high molecular weight polyethylene (UHMW PE) and high density polyethylene (HDPE) powders to impart a hydrophilic (water-wettable) surface to these normally hydrophobic materials. (See Figure 1). This change in compatibility, as well as adhesive bonding capability, enables these treated powders to be sold as performance additives for use in polyurethanes, epoxies, rubbers, unsaturated polyesters and even water-borne systems.
F/O treatment is long-lasting and frequently permanent. This is because the treatment chemistry can penetrate considerably deeper than other surface preparation processes. The longevity is important because it avoids the need to use treated items quickly and enables treated substrates to be inventoried for long time periods. Contrary to common belief, use of highly reactive chemicals is actually environmentally friendly. This is because highly reactive chemicals are readily and essentially quantitatively neutralized because they are so reactive. Processes that use unreactive chemicals are some of the most difficult to scrub or neutralize. The spent solid state scrubbing medium used to neutralize the F/O process has commercial value for other manufacturing firms and is sold as a reagent.
F/O surface preparation is used in a wide range of decorating and finishing product applications, such as nylon sporting goods to facilitate better overmolding processing with soft-grip materials and in the automotive industry to facilitate overmolding with dissimilar durometer polyurethanes, including direct casting of polyurethane tires. Similar to gas phase surface oxidation processes, F/O also has difficulty treating highly fluorinated materials such as PTFE (TeflonŽ).
Inhance/Fluoro-Seal LLC., Houston, TX, is the developer of this technology. As with all surface pretreatment processes and chemical primers, its production use is application specific. Fluorooxidation is a safe and reliable process offering a new alternative for inline pretreatment.
Scott Sabreen is the founder and president of The Sabreen Group, Inc., a plastics engineering consulting firm. He is a board member for the Society of Plastics Engineers Decorating/Assembly Division, technical editor for Plastics Decorating and expert engineer for Omnexus/SpecialChem, Intota-Guideline and Nerac. Sabreen may be reached via email at [email protected].