SURFACE ACTIVATION AND GRAFTING

Plasma activation is the alteration of surface characteristics by the substitution or addition of new chemical groups from active species created in a plasma for groups normally present in the base polymer. For example, conventional polyethylene can be made more useful by transforming its surface with simple plasma treatments as shown in Figure 2. Such groups become 'handles" that can perform new roles. For example, hydroxyl and carboxylic acid groups can be grafted to polyethylene to make the once hydrophobic surface hydrophilic. Amide and amine groups could alternatively be grafted to make surfaces receptive to dyes for coloration.

Process gases such as 0₂, N₂, He, Ar, NH₃, N₂0, C0₂, CF₄, and air or some combination of these gases are generally used in activation treatments. The activation mechanism is believed to be the creation of free radicals on the polymeric material's surface molecules and then subsequent coupling of these free radicals with active species from the plasma environment. Depending on process gas, a large variety of chemical groups can be incorporated into the surface (e.g., hydroxyl, carbonyl, carboxylic, amino or peroxyl groups).

To better understand the complexity of some of the chemistry involved, consider the case of an oxygen plasma. The following oxidation reaction scheme is a logical pathway to produce oxygenated groups grafted on a polymeric surface. First, hydrogen is abstracted from the polymer backbone, R, by atomic oxygen present in the plasma leaving the polymer with a free radical site:

RH + 0• ® R• + •OH

Then, molecular oxygen can couple to the free radical creating a peroxy radical:

R• + O₂ ® RO₂

The peroxy radical can then abstract hydrogen from a neighboring polymer backbone or other source and rearrange into a carboxylic acid group or an ester:

Not indicated in this reaction scheme are the possible formation of alcohols, ethers, peroxides and hydroperoxides. The byproducts, typically C0₂, H₂0 and low molecular weight hydrocarbons, are readily removed by the vacuum pumps.

Additional co-reactants can produce new surface chemistry or accelerate the reaction kinetics. For example, in an oxygen plasma, the breaking of the carbon-carbon and carbon-hydrogen bonds are the rate limiting steps. When tetrafluoromethane is introduced as a co-reactant, the 02/CF4 plasma yields excited forms of 0, OF, CO, CF₃, C0₂, and F. Fluorine and fluorine containing species are more effective in breaking the carbon-carbon and carbon-hydrogen bonds (than oxygen species), thereby accelerating the reaction rate. Oxidation by fluorine free-radicals is known to be as effective as oxidation by the strongest mineral acid etchant solutions, with one important difference: the plasma byproducts do not require special handling. As soon as the plasma is shut off, or the excited species exit the rf field, the species recombine to their original stable and non-reactive form, usually within a few seconds.

As an example of a commercial application, we consider paint adhesion to polymers, an important need in automotive manufacturing. For the painting of plastic surfaces, cleanliness alone is not necessarily sufficient to assure enduring paint adhesion. Rather, grafting of new surface chemistry is needed. Polymers, such as polyolefins and polyolefin alloys, e.g., Thermoplastic Olefins (TPO), are especially difficult to paint due to their "waxy" surface and require pretreatment to provide paint film adhesion. The most common pre-treatment for TPO prior to painting has been either application of oxy/acetylene flames directly onto the surface (flaming) or the application of chemical adhesion promoters. Flaming, while effective to a degree, is not practical with more sophisticated panel designs which have recesses, louvers, or deep accent grooves. Adhesion promoters, typically low-solids (<5%) solution of chlorinated olefins in solvent, generally provide a higher level of effectiveness than flame treatment. Solvent-based adhesion promoters are not environmentally friendly since they contain a large proportion of volatile organic compounds. Water-borne adhesion promoters have not yet proven to be as effective and are more costly. Plasma treatment outperforms these commercial pre-treatment processes or combinations of processes^3,4. Paint adhesion has actually been shown to exceed the strength of the TPO base material, which has never been demonstrated with any other pretreatment process. Material was treated with an air feed gas at 0.2-0.4 mTorr, rf energy density from 0.01 to 0.1 W/cm² and treatment time from 30 to 60 sec. Plasma treatment provided a 1400 to 1800% improvement in peel strength vs the control, while the failure mechanism shifted from adhesive between the paint-substrate interface to cohesive within the TPO substrate.

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