Лако-красочные материалы — производство

The use of plasma surface treatment to improve adhesion is well known [5-19] and several literature sources provide an in-depth discussion of the nature of gas plasmas and their chemistries [1,5,7,10-12]. Although any gas can be ionized using RF excitation, gases such as O2, N2, He, Ar, NH3, N2O, CO2, CF4, and air or some combination of these gases are generally used for surface treatment.

One of the more common plasma processes used to enhance the adhesion of polymers is surface treatment in an oxygen plasma. An oxygen plasma is aggressive in its reactivity and forms numerous components. Within an oxygen plasma O+, O“, O+, O2, O, O3, ionized ozone, metastably excited O2, and free electrons are generally observed. The ionization of oxygen into the various species found in an oxygen plasma can be repre­sented by the following reaction scheme:

O2 + e — O2 — O + O2 e + O — O+ + 2e e + O2 — O+ + 2e e + 2O2 —— O2 + O2 O2 + O — O3 + e

These reactions represent a small sampling of those that occur in an oxygen glow discharge.

As the components formed during the ionization recombine, they release energy and photons, emitting a faint blue glow and much UV radiation. The photons in the UV region have enough energy to break the carbon-carbon and carbon-hydrogen bonds in the materials on the surface that are exposed to the plasma. In the case of contaminants, the net effect appears to be degradative, such that lower-molecular-weight materials are created. These lower-molecular-weight materials are subsequently removed by the vacuum. In this manner the surface that has been exposed to a plasma is cleaned. Lower-molecular-weight polymer fractions that comprise the weak boundary layers on the surface are also removed in this manner. Several reports documenting the efficacy of plasma surface cleaning have been published [1,13,15,20,21].

Once the contaminants have been removed, the virgin polymer surface is exposed to the plasma environment. The electrons, ions, and free radicals in the plasma act on this exposed polymer, creating free radicals in the molecular chains on the surface [l,22,23]. The free radicals that are created on the polymer surface by this process can then react with the various molecular and active species present in the plasma environment. In a low — pressure oxygen plasma, the following oxidation reaction scheme has been suggested:

RH + O* — R* + * OH

R T O2 —— RO2

RO2* + RH — RO2H + R0*

RO2* + R0* — RO2R0

Here the RO2H and RO2R0 indicate the formation of acids and esters. Not indicated in this reaction scheme are the possible formation of alcohols, ethers, peroxides, and hydro­peroxides.

Thus in addition to the reactions resulting from the bombardment of the surface by photons, ions, and neutral particles, all of the active species in the plasma react with the polymer surface. The by-products, consisting of CO2, H2O, and low-molecular-weight hydrocarbons are readily removed by the vacuum system. The use of co-reactants can serve to modify the surface chemistry obtained with a single gas chemistry or to accelerate the reaction kinetics. For example, in an oxygen plasma, breaking of the carbon-carbon and carbon-hydrogen bonds is the rate-limiting step. When tetrafluoromethane is intro­duced as co-reactant, the O2/CF4 plasma yields excited forms of O, OF, CO, CF3, CO2, and F. Since fluorine or fluorine-containing species are more effective in breaking the carbon-carbon and carbon-hydrogen bonds, the reaction rate is accelerated. The perma­nent nature of these changes on the polymer surface has been confirmed by spectroscopic analyses and documented in several studies [24-27]. The use of other gases permits incor­poration of other functional groups on the polymer surface. Examples include the use of ammonia, nitrogen, and oxides of nitrogen plasmas that are used to incorporate nitrogen in the surface and create nitrogen-based functional groups such as primary and secondary amines [28,29].

One result of such surface modification of the polymer surface is an increase in the surface energy of the polymer and an attendant improvement in surface wetting. As stated earlier, adequate wetting of the surface by the adhesive contributes to the improvement in bond strength by increasing the apparent area of contact over which the load is distrib­uted. Published studies suggest that this improvement in wetting contributes directly to the observed improvement in the strength of the adhesive bond [30-32]. Another factor that contributes to improved adhesion is an increase in surface area of the polymer surface through microroughening. This occurs through the process of ablation of the polymer surface through exposure to a plasma. This is particularly the case when the plasma is highly reactive, as in the case when oxygen is used as one of the gas components that is being ionized. The nature of the gas being ionized to create the plasma is not the only factor that determines the extent of ablative etching. The nature of the polymer that is exposed to the plasma also plays a key role. Studies have shown that etching through ablation of surface polymer layers does occur in the case of polymers such as polyethylene (PET) and nylon 66 [11,33], whereas polyaramid materials such as Kevlar appear to be resistant to microroughening through ablation of the polymer chains [34].

Evidence has been presented in several studies which indicates that the strength of the adhesive bond is dependent on the particular functional group that has been created on the surface of the polymer. In some cases a direct correlation is drawn relating the nature of the chemical groups on the surface, the nature of the adhesive used, and the observed improvement in adhesion [11,32,35]. In other cases, the improvements are related to the effects of hydrogen bonding and specific surface chemical interactions that do not neces­sarily result in covalent bonding between the polymer surface and the adhesive [36]. The reader may infer these conclusions from the adhesion data presented along with the data describing the nature of the surface chemistry as determined by X-ray photoelectron spectroscopy (XPS) analysis [37,38].

As these examples illustrate, selection of the process gas determines how the plasma will alter the polymer. Very aggressive plasmas can be created from relatively benign gases. Oxidation by fluorine free radicals that are generated when tetrafluoromethane is included as one of the gases is as effective as oxidation by the strongest mineral acid solution. The primary difference is that the by-products of the plasma process do not require special handling since the active species recombine to their original stable and nonreactive form outside the RF field. In all cases, profound changes in the chemical nature of the polymer surface are implemented, changes that are permanent in nature. The stability of these surface changes is a function of the materials themselves and the storage conditions used [39]. For instance, plasticizers that can migrate to the surface or contaminants in the storage area that can be attracted to these high-energy surfaces will negate the effects of the chemical changes that have been created on the surface of these materials. Contact — angle measurements and electron spectroscopy for chemical analyses of plasma-treated surfaces have confirmed the permanent and long-lasting nature of plasma surface mod­ification of polymers. For example, plasma-modified fluorinated ethene propene (FEP) was shown to retain its surface chemical characteristics over an 18-month observation period [40]. Similar phenomena have been observed by other investigators for other mate­rials, such as polyethylene and polystyrene (unpublished data, HIMONT Plasma Science Applications Laboratory). These changes ultimately lead to significant improvements in adhesion strength, as the data in Table 1 suggest.